Latest / Daniel and Kelly’s Extraordinary Universe / Why are some things transparent?

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Why are some things transparent? · Daniel and Kelly’s Extraordinary Universe

• 0:08
  Speaker 1: Hey, or hey, have you ever seen a glass frog?
• 0:12
  Speaker 2: Mmm?
• 0:12
  Speaker 3: You mean one of those transparent amphibians or is it
• 0:15
  Speaker 3: like a it'll keep saking me ou out of class?
• 0:18
  Speaker 1: I mean they're a real live frog. I was reading
• 0:21
  Speaker 1: that they are native to Central America, including Panama, where
• 0:24
  Speaker 1: you grew up. Yeah.
• 0:25
  Speaker 3: I have heard that, but to be honest, I haven't
• 0:27
  Speaker 3: seen one in person. You could probably find them in
• 0:30
  Speaker 3: the jungle, but it's not like they're jumping around my house.
• 0:33
  Speaker 1: Well did you ever wonder what it would be like
• 0:35
  Speaker 1: to be transparent yourself?
• 0:37
  Speaker 3: Sounds terrible? I guess you want people to see, right.
• 0:42
  Speaker 3: Everyone wants to be seen these.
• 0:43
  Speaker 1: Days, unless you want to sneak around the house and
• 0:45
  Speaker 1: be invisible.
• 0:46
  Speaker 3: I guess if you have the option of turning transparent,
• 0:49
  Speaker 3: that's cool. Like, I think all kids dream of being
• 0:52
  Speaker 3: invisible at some point.
• 0:53
  Speaker 1: I always wondered what happens if you're invisible and you
• 0:55
  Speaker 1: take a bite of an apple, Like, can everybody see
• 0:58
  Speaker 1: that apple work its way through you? H?
• 1:00
  Speaker 3: I guess it depends on you, know, like how the
• 1:03
  Speaker 3: invisibility works, Like are you a space that is transparent
• 1:07
  Speaker 3: or is it just your molecules are transparent? Or you
• 1:09
  Speaker 3: can just eat a cookie mid out of glass frogs
• 1:11
  Speaker 3: or eat glass. I guess that sounds less tasty.
• 1:14
  Speaker 1: I don't think I want to take a bite out
• 1:16
  Speaker 1: of that.
• 1:16
  Speaker 3: Yeah, I see you, man, I see you. I am Rhem,
• 1:34
  Speaker 3: a cartoonist and the creator of PhD comics.
• 1:36
  Speaker 1: Hi, I'm Daniel. I'm a particle physicist and a professor.
• 1:39
  Speaker 1: You see Irvine, And even though you can only hear
• 1:42
  Speaker 1: my voice, I feel seen.
• 1:46
  Speaker 3: Well, that's good. Everyone wants to be seen. I think
• 1:49
  Speaker 3: mostly we just feel heard in this podcast.
• 1:51
  Speaker 1: That's right. We feel heard, not hurt, because we love
• 1:54
  Speaker 1: sharing with you our passion and curiosity about the universe.
• 1:57
  Speaker 1: And when I hear back from listeners that's something we
• 2:00
  Speaker 1: have said has touched on their deep seated need to
• 2:03
  Speaker 1: understand the universe. I do feel heard and seen.
• 2:06
  Speaker 3: Yeah, because it is a pretty amazing universe with lots
• 2:09
  Speaker 3: to see out there and to hear. I guess if
• 2:11
  Speaker 3: you have the right kinds of ears. Because sound doesn't
• 2:14
  Speaker 3: travel in space, does it?
• 2:15
  Speaker 1: It actually does, just very very slowly.
• 2:19
  Speaker 3: Why are you kidding or are you serious?
• 2:21
  Speaker 4: No?
• 2:21
  Speaker 1: Space is not totally empty, and so in principle, there
• 2:24
  Speaker 1: are sound waves that do propagate through the solar wind,
• 2:28
  Speaker 1: for example. But in practice, if you're in outer space,
• 2:30
  Speaker 1: you're going to freeze before you hear anything.
• 2:32
  Speaker 3: Also, you would be hearing it in slow motion, I
• 2:34
  Speaker 3: guess right, if sound moves slower, Like, don't go out
• 2:37
  Speaker 3: in space too late, you're.
• 2:39
  Speaker 1: Dead, that's right. You should shout at your friends in space,
• 2:44
  Speaker 1: like a year before they go space walking without a
• 2:47
  Speaker 1: sud on so they can hear you.
• 2:49
  Speaker 3: Yeah, oh, that's not going to help them. Also, how
• 2:53
  Speaker 3: do you shout in space? Like you have to take
• 2:54
  Speaker 3: off your helmet to shout, and then that's not good.
• 2:57
  Speaker 3: Either in space nobody can hear you. Or you shouldn't
• 3:00
  Speaker 3: be hurt or you might get hurt.
• 3:02
  Speaker 1: But you should turn your eyeballs up to the night
• 3:04
  Speaker 1: sky and wonder how everything works. And you should also
• 3:07
  Speaker 1: look down on the ground beneath you and see if
• 3:09
  Speaker 1: you can puzzle out the mysteries of everyday objects, the
• 3:13
  Speaker 1: nature of our universe, how everything works, how it weaves
• 3:15
  Speaker 1: itself together to make this incredible cosmos. That's one of
• 3:19
  Speaker 1: the deep mysteries that humans want to unravel. And those
• 3:21
  Speaker 1: are the topics we like to take a part on
• 3:23
  Speaker 1: this podcast.
• 3:24
  Speaker 3: Yeah, and It's amazing and lucky that we can see
• 3:27
  Speaker 3: so many things out there in the universe and around us,
• 3:29
  Speaker 3: so that we know where they are and we can
• 3:31
  Speaker 3: also study them and figure out how they work, what
• 3:33
  Speaker 3: they're made out of, and what the rules of this
• 3:36
  Speaker 3: crazy universe are. But anyways, welcome to our podcast, Daniel
• 3:40
  Speaker 3: and Jorge Explain the Universe, a production of iHeartRadio.
• 3:43
  Speaker 1: In which we try to do exactly that, take apart
• 3:45
  Speaker 1: the whole universe, see how it works, and explain it
• 3:49
  Speaker 1: to you. Some of the questions we love to tackle
• 3:51
  Speaker 1: involve the tiniest things in the universe, what the rules
• 3:54
  Speaker 1: are for how they work, or the biggest things in
• 3:56
  Speaker 1: the universe, like the whole universe itself or super massive
• 4:00
  Speaker 1: black holes at the hearts of galaxies. But there's also
• 4:03
  Speaker 1: a lot of fascinating physics in between. How those tiny
• 4:06
  Speaker 1: little objects pull themselves together to behave in weird and
• 4:09
  Speaker 1: wonderful ways. The reason ice cream is so tasty, the
• 4:12
  Speaker 1: reason metals conduct electricity, the reason your chair holds you
• 4:16
  Speaker 1: up is because of zillions of atoms all working together
• 4:19
  Speaker 1: to have fascinating phenomena.
• 4:21
  Speaker 3: Yeah, and thankfully we can see them all and hear
• 4:24
  Speaker 3: them all so we can study them. It'd be kind
• 4:27
  Speaker 3: of hard to know and study the universe if everything
• 4:29
  Speaker 3: was invisible, right, or if we didn't have eyeballs.
• 4:31
  Speaker 1: In years, that's true, and recently we've discovered the amazing
• 4:34
  Speaker 1: fact that most of the universe is invisible. The dark
• 4:38
  Speaker 1: matter that's out there is most of the stuff in
• 4:40
  Speaker 1: the universe, and we didn't even know it existed until recently.
• 4:44
  Speaker 1: Because it's invisible to us. Life passes right through it,
• 4:48
  Speaker 1: and there's lots of things going on out there in
• 4:50
  Speaker 1: the universe that you just cannot see because your eyeballs
• 4:53
  Speaker 1: can't pick them up, you can't taste them, you can't
• 4:55
  Speaker 1: smell them. So most of the universe is actually invisible.
• 4:59
  Speaker 1: Transparency turns out to be the name of the game.
• 5:02
  Speaker 3: So the universe got its kid wish of being invisible.
• 5:05
  Speaker 3: Is that what you're saying? What's it trying to sneak
• 5:07
  Speaker 3: around and do?
• 5:07
  Speaker 1: I don't know, but those dark matter kids are probably
• 5:09
  Speaker 1: eating dark matter cookies all night long, and.
• 5:14
  Speaker 3: They're made out of dark chocolate and taste better too.
• 5:17
  Speaker 3: And my new wish used to be a dark matter person.
• 5:21
  Speaker 3: They get to eat dark meat and their chicken.
• 5:24
  Speaker 1: Well, It also answers that other question. You know what
• 5:26
  Speaker 1: happens when a dark matter kid eats a dark matter cookie.
• 5:29
  Speaker 1: Of course it goes through them and becomes dark matter
• 5:32
  Speaker 1: on the way out.
• 5:32
  Speaker 3: Darker matter, you mean, the darkest matter, because a black hole.
• 5:38
  Speaker 3: That's where black holes come from.
• 5:40
  Speaker 1: That's exactly right. Black holes are dark matter toilets.
• 5:43
  Speaker 3: They're are worse through the things that go into the toilet.
• 5:47
  Speaker 1: Here we are elevating the discourse of the nature of
• 5:49
  Speaker 1: the universe.
• 5:50
  Speaker 3: Yeah, I mean, listeners want to be reached into their
• 5:52
  Speaker 3: soul and touched right and seen and hurt.
• 5:56
  Speaker 1: I'm not sure they want to have this particular taste.
• 5:58
  Speaker 3: However, Well, who said anything about tasting.
• 6:02
  Speaker 1: We're talking about eating dark matter cookies. You're fantasizing about
• 6:05
  Speaker 1: dark matter chocolate over there. It's all about taste.
• 6:07
  Speaker 3: That's why we're all about taste here.
• 6:08
  Speaker 1: Good taste, bad taste, you.
• 6:10
  Speaker 3: Decide, that's right. We're we're scientists. We explore the full
• 6:14
  Speaker 3: range of tastes available to the human experience. But it
• 6:17
  Speaker 3: is interesting that even the things that are invisible, we
• 6:20
  Speaker 3: can still somehow see them through other things. Right, I mean,
• 6:23
  Speaker 3: if dark matter, dark energy was completely invisible. We wouldn't
• 6:27
  Speaker 3: even know it was there, but somehow it has an
• 6:29
  Speaker 3: influence on things that we can see, thankfully, and that's
• 6:31
  Speaker 3: how we know they're there.
• 6:32
  Speaker 1: Yes, transparency turns out to be quite a subtle issue.
• 6:36
  Speaker 1: Some things can be transparent to one kind of light
• 6:38
  Speaker 1: but not to another. Some kinds of light can pass
• 6:41
  Speaker 1: through some objects but not other objects. As always, when
• 6:44
  Speaker 1: you dig into it, you discover there's a fascinating physics
• 6:47
  Speaker 1: story that underlines how things work.
• 6:49
  Speaker 3: Yep, this is an interesting topic and so today on
• 6:52
  Speaker 3: the podcast we'll be tackling the question how does transpy work?
• 7:01
  Speaker 1: Some titled how can your kids eat cookies without you knowing?
• 7:04
  Speaker 3: Well, I don't think they need to be transparent to
• 7:06
  Speaker 3: do that, depending how sneaky they are and how how
• 7:10
  Speaker 3: early are you going to sleep or wake up? Now,
• 7:13
  Speaker 3: this is an interesting question, Daniel. I imagine it means
• 7:16
  Speaker 3: what makes things see through? Not like how do transparencies
• 7:19
  Speaker 3: work or how does government transparency work? Because that apparently
• 7:23
  Speaker 3: doesn't work that well or sometimes it works too well.
• 7:26
  Speaker 1: That's right. We want sunlight on all the operations of
• 7:30
  Speaker 1: the universe, but in particular here we're wondering about why
• 7:33
  Speaker 1: light can pass through some kinds of things. Why you
• 7:36
  Speaker 1: can see through glass but you can't see through stone.
• 7:40
  Speaker 1: Why X rays reveal your soft tissues but not your bones.
• 7:43
  Speaker 1: What is the underlying physics? What is the microscopic picture
• 7:47
  Speaker 1: of transparency.
• 7:49
  Speaker 3: We're exploring the full range of transparent behavior.
• 7:51
  Speaker 1: We're just trying to be transparent about how the universe works.
• 7:53
  Speaker 3: We're trying to be transparent about how the podcast works,
• 7:56
  Speaker 3: is what I'm saying. And to be fully transparent, I
• 7:59
  Speaker 3: have not I've read the outlete for today's episode. I
• 8:03
  Speaker 3: am kind of winging it today, which is totally different
• 8:05
  Speaker 3: from other days.
• 8:06
  Speaker 1: Right exactly. If you don't notice the difference between this
• 8:10
  Speaker 1: episode and any other episode, then folks, you learn something
• 8:13
  Speaker 1: about how the sausage is made.
• 8:16
  Speaker 3: You learned that Jorges could thinking done the fly and
• 8:19
  Speaker 3: reading really quickly. But anyways, as usually, we were wondering
• 8:22
  Speaker 3: how many people out there had thought about things that
• 8:24
  Speaker 3: are transparent and if they know how transparency works.
• 8:28
  Speaker 1: So thanks very much to everybody who answers these questions.
• 8:30
  Speaker 1: It gives us a great insight into what people already
• 8:33
  Speaker 1: know and don't know. If you'd like to help us
• 8:35
  Speaker 1: out for a future episode of the podcast. Please don't
• 8:38
  Speaker 1: be shy write to us two questions at Danielandjorge dot com.
• 8:42
  Speaker 3: So think about it for a second. Do you know
• 8:44
  Speaker 3: how transparency works? Here's what people had to say.
• 8:47
  Speaker 4: Transparency works, in my opinion, by not refracting, or the
• 8:53
  Speaker 4: medium that the lightest passing through does not refract in
• 8:57
  Speaker 4: any way or change the path of the life. And
• 9:01
  Speaker 4: that way you don't get any kind of interference and
• 9:05
  Speaker 4: you can see through something it is transparent, so perfectly clear.
• 9:10
  Speaker 2: I'm pretty sure that transparency works by like the light
• 9:13
  Speaker 2: can travel through the material without being absorbed, so maybe
• 9:17
  Speaker 2: there's a lot of spaces between it, and so yeah,
• 9:20
  Speaker 2: that light just doesn't get absorbed, so it goes through
• 9:24
  Speaker 2: and it's merry way.
• 9:25
  Speaker 5: At first, I thought it was simple because the material
• 9:27
  Speaker 5: density would lead to more or less transparency, like air
• 9:31
  Speaker 5: or some gas and low density would be transparent to
• 9:34
  Speaker 5: some point because of the particle density. But then again,
• 9:37
  Speaker 5: glass is quite a high density, right, and it's still transparent,
• 9:41
  Speaker 5: So maybe it's about reflective index. And yeah, like if
• 9:46
  Speaker 5: you put glass in a water that comes invisible because
• 9:48
  Speaker 5: it's fully transparent because of the reflective indices, I.
• 9:51
  Speaker 6: Would guess transparency has to do with interactions between different
• 9:56
  Speaker 6: forces and particles and whether they interact or not. If
• 9:59
  Speaker 6: they interact, they are not transparent to each other, and
• 10:02
  Speaker 6: if they don't interact, they're transparent to each other.
• 10:04
  Speaker 1: It's like a real a race.
• 10:09
  Speaker 7: So photon gets emitted, it's absorbed by the first atom
• 10:14
  Speaker 7: in the glass, for instance, and then it gets retransmitted
• 10:18
  Speaker 7: to the next, and so on and so forth until
• 10:21
  Speaker 7: it goes over the other side of the of the
• 10:23
  Speaker 7: glass or whatever transparent material we are talking about.
• 10:28
  Speaker 8: In order to reflect light, the photon would have to
• 10:30
  Speaker 8: be absorbed and then re emitted, I think so. I
• 10:35
  Speaker 8: guess in order to be transparent, it would have to
• 10:37
  Speaker 8: be something that doesn't absorb photons for whatever reason. That'd
• 10:43
  Speaker 8: be interesting one.
• 10:43
  Speaker 3: That all right, some pretty intricate answers. I feel like
• 10:47
  Speaker 3: we had some hardcore physicists here in the pool today.
• 10:50
  Speaker 3: Did you ask people on the internet or in your department?
• 10:53
  Speaker 1: These are all Internet answers, and they really reflect the
• 10:56
  Speaker 1: incredible complexity of light and matter into it in a
• 11:00
  Speaker 1: few episodes, what happens when things reflect? What color means.
• 11:04
  Speaker 1: But there really is a whole lot going on here.
• 11:06
  Speaker 1: Light bouncing off of matter, or passing through matter, or
• 11:09
  Speaker 1: refracting through matter is really a very complicated phenomenon, very
• 11:13
  Speaker 1: tricky to understand from the microphysics point of view.
• 11:17
  Speaker 3: Yes, I like how it reflects how smarter listeners are.
• 11:21
  Speaker 1: But so many really interesting answers, a lot of which
• 11:24
  Speaker 1: really get at the heart of the question, which is
• 11:26
  Speaker 1: that it's about the interactions of the photons with the material.
• 11:30
  Speaker 1: There's also some misunderstanding there, like the idea that light
• 11:34
  Speaker 1: goes through things if there are spaces for it to
• 11:36
  Speaker 1: like wiggle its way through. That's not really the way
• 11:38
  Speaker 1: that we think about it, but we'll dig into it
• 11:40
  Speaker 1: and explain it.
• 11:40
  Speaker 3: I guess it depends, like the difference between transparency and
• 11:43
  Speaker 3: air and transparency in glass and transparency through different kinds
• 11:48
  Speaker 3: of light, right, it sort of depends.
• 11:50
  Speaker 1: There are definitely differences between air and glass, but the
• 11:52
  Speaker 1: basic physics of transparency is quite similar in either case.
• 11:56
  Speaker 1: Are the photons like avoiding the atoms, It's not like
• 11:59
  Speaker 1: they're fine a pathway through. That's maybe the way transparency
• 12:03
  Speaker 1: works for like a screen door. You can see through
• 12:05
  Speaker 1: a screen because there are literally holes that a photon
• 12:08
  Speaker 1: can pass through. But the reason you can see glass
• 12:10
  Speaker 1: is not because it's like a microscreen. That glass is
• 12:13
  Speaker 1: finding tiny little holes in the glass to wiggle its
• 12:16
  Speaker 1: way through.
• 12:17
  Speaker 3: I guess we'll dig into that. And so let's jump
• 12:19
  Speaker 3: right in, Daniel, what are the basics what makes something transparent?
• 12:22
  Speaker 1: So first let's clarify what we mean by transparent. Right,
• 12:25
  Speaker 1: by transparent really just mean that you can see through it.
• 12:28
  Speaker 1: It means if you shine a light from one side,
• 12:31
  Speaker 1: the light comes out the other side with basically no scattering,
• 12:34
  Speaker 1: Like the light is still coherent. If you have an
• 12:36
  Speaker 1: image of an ice cream cone on one side, you're
• 12:38
  Speaker 1: going to see the ice cream cone still on the
• 12:41
  Speaker 1: other side. It might be refracted a little bit or
• 12:43
  Speaker 1: bent or distorted, but it's transparent if those images are preserved.
• 12:47
  Speaker 3: I see. So like, if light can pass through it
• 12:49
  Speaker 3: basically is what it means to be transparent, or information
• 12:52
  Speaker 3: from light passes through it without any distortions exactly.
• 12:56
  Speaker 1: And even glass, which we think of as transparent, does
• 12:58
  Speaker 1: distort it a little bit. Like if you stick your
• 13:00
  Speaker 1: finger behind a pane of glass and only half of
• 13:03
  Speaker 1: your finger is sticking behind it, you'll notice the finger
• 13:05
  Speaker 1: no longer looks like a whole finger. It's like broken
• 13:07
  Speaker 1: in half because the path of the light through the
• 13:10
  Speaker 1: glass changes a little bit relative to the path of
• 13:12
  Speaker 1: the light through the air. That's refraction, which is a
• 13:14
  Speaker 1: whole other complicated topic that we dug into in another episode.
• 13:18
  Speaker 1: But still you can see your finger right the same
• 13:19
  Speaker 1: way you can see through water and you can see
• 13:22
  Speaker 1: through air. You can identify things. You can recognize things
• 13:25
  Speaker 1: even if the path of the light is slightly.
• 13:27
  Speaker 3: Changed, all right. So that means that a light can
• 13:30
  Speaker 3: or at least information that the light had before it
• 13:32
  Speaker 3: went through the material makes it through unscales. That's what
• 13:35
  Speaker 3: it means to be transparent.
• 13:36
  Speaker 1: Yeah, And the fundamental thing that's happening there relates to
• 13:39
  Speaker 1: how photons either interact or don't interact with the electrons
• 13:45
  Speaker 1: in that material. And to understand that, we have to
• 13:48
  Speaker 1: understand a little bit about the quantum mechanics of those electrons,
• 13:51
  Speaker 1: what energy is they're allowed, and how photons decide whether
• 13:54
  Speaker 1: or not to interact with those electrons.
• 13:56
  Speaker 3: And you were saying, it's not like it's a screen
• 13:59
  Speaker 3: door where like some other light is finding a pass
• 14:02
  Speaker 3: through it, But that can also happen, Kennet, Like if
• 14:05
  Speaker 3: you have something really thin maybe, or something really sparse
• 14:10
  Speaker 3: or light like air, there has to be some photons
• 14:12
  Speaker 3: that are making it through without interacting with anything. Right.
• 14:15
  Speaker 1: Well, most materials, even like a thin sheet of paper,
• 14:18
  Speaker 1: are dense enough that photons are not like finding holes
• 14:21
  Speaker 1: in them. The reason that light gets through is because
• 14:25
  Speaker 1: the photons are ignoring the atoms. It's not that they're
• 14:28
  Speaker 1: missing them, it's that they're passing through without interacting. Because
• 14:31
  Speaker 1: when a photon passes by an atom, it doesn't always
• 14:34
  Speaker 1: interact with it. There are rules about whether or not
• 14:37
  Speaker 1: photons can interact with atoms, and in a solid it's
• 14:39
  Speaker 1: complicated because you have lots and lots of atoms. But
• 14:42
  Speaker 1: the easiest way to understand it is to start with
• 14:44
  Speaker 1: an individual atom. Just think about like a single hydrogen
• 14:47
  Speaker 1: atom in space and you shoot a photon at it
• 14:49
  Speaker 1: is that photon going to get absorbed by the atom
• 14:52
  Speaker 1: or is the photon going to ignore the atom? And
• 14:54
  Speaker 1: that depends on the energy of the photon, because these
• 14:58
  Speaker 1: atoms can only absorb photon of certain energies.
• 15:01
  Speaker 3: Well, I guess it also maybe depends on where you
• 15:03
  Speaker 3: shoot the photon, right, Like, if I have an hydrogen atom,
• 15:06
  Speaker 3: floating out there in the air and I shoot a
• 15:09
  Speaker 3: laser beam, you know, a mile to one side of it.
• 15:12
  Speaker 3: It's not going to interact with the hydrogen, is it.
• 15:14
  Speaker 1: No, that's right. But if you have a huge wall
• 15:17
  Speaker 1: of hydrogen atoms and you shoot the laser beam at it,
• 15:19
  Speaker 1: then it's not going to be able to avoid the hydrogen, right,
• 15:23
  Speaker 1: It's going to have to get through the other side.
• 15:24
  Speaker 1: It's going to have to pass through the hydrogen. And
• 15:27
  Speaker 1: that's what makes things transparent. I mean, one thing is
• 15:30
  Speaker 1: to have holes in it. Sure, where you can see
• 15:31
  Speaker 1: through a screen door, even if it's made of metal
• 15:34
  Speaker 1: or stone, as long as there are holes in it.
• 15:36
  Speaker 1: But I think the physics of transparency is more interesting
• 15:39
  Speaker 1: if we think about what happens when light ignores the material,
• 15:43
  Speaker 1: if it passes through the material, rather than finding holes
• 15:47
  Speaker 1: or ways around it.
• 15:48
  Speaker 3: Well, I know it's more interesting to you as a physicist,
• 15:52
  Speaker 3: but I guess for me, I'm just curious about which
• 15:54
  Speaker 3: has more of an effect to make something transparent, right,
• 15:57
  Speaker 3: Because you know, people always talk about like material like
• 16:00
  Speaker 3: our skin, our bodies, they're made out of atoms. But
• 16:02
  Speaker 3: the atoms are really sort of far apart and the
• 16:04
  Speaker 3: nucleus is really far apart from the electrons, So there's
• 16:07
  Speaker 3: a lot of sort of like supposedly empty space, even
• 16:10
  Speaker 3: within solid matter like ourselves. I'm just wondering, like, you know,
• 16:14
  Speaker 3: there's it seems like there's a lot of space for
• 16:16
  Speaker 3: a light to squeeze through or to pass by without
• 16:20
  Speaker 3: even knowing there are other things there.
• 16:22
  Speaker 1: That's an interesting point, and I think a lot of
• 16:24
  Speaker 1: people have maybe the wrong mental picture of what an
• 16:26
  Speaker 1: atom sort of looks like to a photon. You know,
• 16:29
  Speaker 1: even if you have a grid of atoms, like a
• 16:30
  Speaker 1: sheet of atoms making them material, you might imagine that
• 16:33
  Speaker 1: it's mostly empty space, as you said. And it's true
• 16:35
  Speaker 1: that the nucleus is small compared to the distances between
• 16:38
  Speaker 1: the atoms, right, So you have this like grid of atoms,
• 16:41
  Speaker 1: and there is a lot of space between the nuclei.
• 16:44
  Speaker 1: But that space is not empty, right. That space is
• 16:46
  Speaker 1: filled with electrons and electronic fields and forces that are
• 16:51
  Speaker 1: holding the nucleus and the electrons together. So the electrons
• 16:54
  Speaker 1: are in these buzzy clouds all around the nucleus, and
• 16:57
  Speaker 1: the electrons weave the atoms together, right, So it's not
• 17:00
  Speaker 1: like there's space between the atoms. The atoms are held together,
• 17:04
  Speaker 1: they're in these bonds that tie them together into a
• 17:07
  Speaker 1: big grid, into a big lattice, and the electrons can
• 17:10
  Speaker 1: sometimes slide back and forth and move between the atoms,
• 17:13
  Speaker 1: so there isn't a lot of space between the atoms.
• 17:15
  Speaker 1: And then inside the atom, all that space you imagine
• 17:18
  Speaker 1: might be empty is really filled with electrons. So from
• 17:21
  Speaker 1: the point of view of a photon hitting like a
• 17:23
  Speaker 1: sheet of material, a sheet of paper or a sheet
• 17:27
  Speaker 1: of glass or whatever, it really is hitting a wall
• 17:29
  Speaker 1: of electrons that it can't find a way around unless
• 17:33
  Speaker 1: you like physically punch holes in it.
• 17:35
  Speaker 3: You just made me wonder, like when you have a material,
• 17:37
  Speaker 3: how close together are the electrons shells between atoms, Like
• 17:43
  Speaker 3: are are they bumping up against each other or is
• 17:45
  Speaker 3: there a certain amount of space between.
• 17:48
  Speaker 1: It depends a little bit on the kind of atom
• 17:49
  Speaker 1: you have. But atoms tend to bond with their outermost electrons,
• 17:53
  Speaker 1: and we'll dig into that if you let me talk
• 17:55
  Speaker 1: about how these electrons move between the atoms. But you
• 17:59
  Speaker 1: really are tying the outer levels of the electron orbitals
• 18:02
  Speaker 1: together and then informing these common electron energy levels. So
• 18:06
  Speaker 1: these atoms are sharing electrons that really are woven together.
• 18:10
  Speaker 3: I see. I think what you're saying is that to
• 18:11
  Speaker 3: a photon a grid of electrons, it's like a solid
• 18:15
  Speaker 3: wall almost because it's full of electron fields, and photons
• 18:19
  Speaker 3: interact with electrons. But like, if you were something else
• 18:22
  Speaker 3: that was not an electron, you would maybe see a
• 18:24
  Speaker 3: lot of empty space. But because you are a photon,
• 18:26
  Speaker 3: then you're walking into a solid wall of electron fuzziness.
• 18:30
  Speaker 1: Yeah, if you're a neutrino, for example, and you don't
• 18:34
  Speaker 1: feel electric charge, then the fact that there are electromagnetic
• 18:37
  Speaker 1: fields all through these materials is relevant to you, and
• 18:40
  Speaker 1: you pass right through it. So like even a block
• 18:44
  Speaker 1: of lead is basically transparent to a neutrino because it
• 18:47
  Speaker 1: doesn't interact with the stuff. So transparency comes down to
• 18:51
  Speaker 1: whether you interact with the material there, not really whether
• 18:54
  Speaker 1: there are holes there. So photons they can either pass
• 18:57
  Speaker 1: through a material or they can interact with it, and
• 19:00
  Speaker 1: that depends on the atomic structure and the energy levels
• 19:04
  Speaker 1: that the electrons have and whether or not they interact
• 19:06
  Speaker 1: with that photon. Even though a photon sees a wall
• 19:09
  Speaker 1: of material, a buzzing blob of electrons in front of it,
• 19:12
  Speaker 1: it can still sometimes pass right through without interacting, and
• 19:16
  Speaker 1: that's what transparency is.
• 19:17
  Speaker 3: Yeah, I guess it can be transparent not just to light,
• 19:19
  Speaker 3: but to other things. Right, Like, isn't there a famous
• 19:22
  Speaker 3: like gold foil experiment that kind of help people figure
• 19:26
  Speaker 3: out the structure of the atom.
• 19:27
  Speaker 1: Yeah, that's right. Ruththerford shot alpha particles, which are helium
• 19:30
  Speaker 1: nuclei and a very thin tissue of gold, and he
• 19:33
  Speaker 1: expected it to mostly just pass through, occasionally get distorted
• 19:37
  Speaker 1: a little bit by forces. What he found was that occasionally,
• 19:40
  Speaker 1: like one in eight thousand times, the alpha particle the
• 19:44
  Speaker 1: helium nucleus bounced right back. Was told him that it
• 19:46
  Speaker 1: was interacting with something hard at the core. So that's
• 19:49
  Speaker 1: what told him that the sheet of gold actually was
• 19:52
  Speaker 1: made out of a grid of hard nuclei, that most
• 19:55
  Speaker 1: of the gold was transparent to the alpha particle, but
• 19:58
  Speaker 1: that occasional little dots of it we're.
• 20:00
  Speaker 3: Not because I guess the helium nuclei would only interact
• 20:04
  Speaker 3: with the nuclei of the gold. Right to the helium particle.
• 20:07
  Speaker 3: It did look like a screen because there is a
• 20:10
  Speaker 3: lot of empty space to a helium atom in between
• 20:14
  Speaker 3: the gold atoms.
• 20:15
  Speaker 1: Yeah, that's mostly true. The nuance there is that the
• 20:17
  Speaker 1: helium does interact with the electrons. It's just that the
• 20:20
  Speaker 1: electrons don't have the mass or the kinetic energy to
• 20:23
  Speaker 1: bounce it back. They can only slightly change its direction.
• 20:26
  Speaker 1: So if the helium goes through the gold and only
• 20:28
  Speaker 1: interacts with the electrons, it gets a little bit of
• 20:30
  Speaker 1: change of direction, whereas if it hits one of the nuclei,
• 20:33
  Speaker 1: those have the mass like push it all the way back.
• 20:36
  Speaker 1: So the sort of two different kinds of interaction the
• 20:38
  Speaker 1: helium can do, one where it punches through and the
• 20:41
  Speaker 1: other one where it bounces back.
• 20:42
  Speaker 3: All right, So you're saying, let's maybe focus on light transparency.
• 20:47
  Speaker 3: And we know that light interacts with electrons, and so
• 20:51
  Speaker 3: material stuff to a photon looks pretty dense and so
• 20:54
  Speaker 3: you can't sort of just go through it without possibly
• 20:57
  Speaker 3: interacting with things. But you're saying, maybe the key transparency
• 21:00
  Speaker 3: is it sometimes like doesn't interact with the thing that's there.
• 21:04
  Speaker 1: That's right. Even if you have a bunch of electrons,
• 21:06
  Speaker 1: photons aren't necessarily able to interact with them. The photon
• 21:09
  Speaker 1: has to have the right amount of energy so the
• 21:12
  Speaker 1: electron can accept it. Can absorb it. If photon has
• 21:15
  Speaker 1: the wrong energy, it'll pass right through without interacting without electron.
• 21:20
  Speaker 1: And that all comes down to how the electrons are
• 21:22
  Speaker 1: confined in the material. I mean a random free electron
• 21:25
  Speaker 1: in space. So you just have like an electron flying
• 21:27
  Speaker 1: through the universe and a photon hits it. It's going
• 21:30
  Speaker 1: to absorb that photon no problem, because electrons flying through
• 21:33
  Speaker 1: space can have any energy, there's no restriction, can have
• 21:36
  Speaker 1: any arbitrary amount of energy, so it will almost always
• 21:40
  Speaker 1: absorb that photon. But an electron around an atom has
• 21:43
  Speaker 1: different rules. Because of the quantum mechanics. It can only
• 21:46
  Speaker 1: exist on like a ladder of energy levels, so it
• 21:49
  Speaker 1: can only absorb photons that move it up one or
• 21:52
  Speaker 1: two or ten steps on that ladder. It can't move
• 21:55
  Speaker 1: up like one and a half steps or two point
• 21:57
  Speaker 1: seven steps, so that limits the kinds of poltons that
• 22:00
  Speaker 1: an electron can absorb.
• 22:02
  Speaker 3: Interesting, it's like the electrons are stuck and they don't
• 22:06
  Speaker 3: want to move from where they are, And so let's
• 22:09
  Speaker 3: get a little bit deeper into that, and then also
• 22:12
  Speaker 3: talk about what happens when something is transparent. So we'll
• 22:14
  Speaker 3: talk about that, but first let's take a quick break.
• 22:29
  Speaker 3: All right, we're talking about transparency here today and what
• 22:32
  Speaker 3: makes things see through. Daniel, you mentioned that it's kind
• 22:36
  Speaker 3: of about how light can go into material, see the material,
• 22:40
  Speaker 3: be near the material, but not interact with it. And
• 22:42
  Speaker 3: you say it has something to do with the energy
• 22:45
  Speaker 3: levels of the electron, because you said that an electron
• 22:48
  Speaker 3: floating out in space will always absorb an electron no
• 22:51
  Speaker 3: matter what does it have to be like flying near it,
• 22:54
  Speaker 3: does it have to hit the electron right in the middle,
• 22:56
  Speaker 3: or does it interact when it's flying nearby? How does
• 22:59
  Speaker 3: that free space case work?
• 23:02
  Speaker 1: If you zap an electron with a photon and that
• 23:04
  Speaker 1: electron is out in free space, it means that there
• 23:07
  Speaker 1: are no rules that govern the energy that the electrons
• 23:10
  Speaker 1: can have. I think something that's really cool and not
• 23:13
  Speaker 1: like widely enough understood, is where quantization comes from, Like
• 23:18
  Speaker 1: why electrons in materials have energy levels. Where these quantum
• 23:23
  Speaker 1: energy levels come from? And it really comes from boundary conditions.
• 23:27
  Speaker 1: It comes from forcing the electron to live within a
• 23:30
  Speaker 1: certain location, like putting it in a box, so that
• 23:33
  Speaker 1: electron out in empty space, it can be here, it
• 23:35
  Speaker 1: can be there, it can have any location. It can
• 23:37
  Speaker 1: have any momentum, and so it's free to absorb a
• 23:40
  Speaker 1: photon of any energy. There's a probability for an electron
• 23:44
  Speaker 1: and a photon to interact. There's still a chance, of course,
• 23:46
  Speaker 1: that a photon will not interact with an electron. It
• 23:48
  Speaker 1: depends on the strength of the force essentially that controls
• 23:52
  Speaker 1: like the probability for these things to happen. But for
• 23:55
  Speaker 1: today's conversation, we can imagine that it basically just always happens.
• 23:58
  Speaker 1: You zap an electron with a photon. It doesn't matter
• 24:02
  Speaker 1: what the energy of that photon is. The electron can
• 24:05
  Speaker 1: accept it because it could have a higher momentum of
• 24:07
  Speaker 1: any value. That's not true for electrons around an atom.
• 24:11
  Speaker 1: It can only have energies of certain values because it's
• 24:14
  Speaker 1: confined into the box of the atom.
• 24:17
  Speaker 3: Now, within that free space electron, when that light hits it,
• 24:20
  Speaker 3: what happens to that electron It gets faster or it
• 24:22
  Speaker 3: gets hotter, and does it start spinning faster? What happens
• 24:25
  Speaker 3: when you use zap an electron in space would.
• 24:27
  Speaker 1: Like, well, it absorbs the momentum of that photon because
• 24:30
  Speaker 1: of conservation momentum, and now it carries that momentum as well.
• 24:33
  Speaker 1: And so if the photon was moving in the same
• 24:36
  Speaker 1: direction as the electron, then it gives it a zip.
• 24:38
  Speaker 1: It's going faster. If the photon was moving in the
• 24:40
  Speaker 1: opposite direction, the electron hits the photon sort of head on,
• 24:44
  Speaker 1: then it gets slowed down. Right. We talked about like
• 24:47
  Speaker 1: laser cooling once on the podcast. You can use lasers
• 24:50
  Speaker 1: to slow things down. Also if you zap things in
• 24:54
  Speaker 1: the right direction.
• 24:55
  Speaker 3: Yeah, that was pretty cool. And so it's sort of
• 24:57
  Speaker 3: like a billier ball. I guess, like if you have
• 24:59
  Speaker 3: an electron other and you hit it with a photon,
• 25:01
  Speaker 3: it basically happens like it does when you hit a
• 25:04
  Speaker 3: billier ball, right with the white ball.
• 25:05
  Speaker 1: Yeah, the quantum mechanics comes in with the probability there's
• 25:08
  Speaker 1: like a chance that the interaction will happen, a chance
• 25:11
  Speaker 1: that it won't. But for today's conversation, we can think
• 25:13
  Speaker 1: of it like a billiard ball. You're giving it a
• 25:14
  Speaker 1: push and it's absorbing that energy. And because a free
• 25:17
  Speaker 1: electron one out in the middle of space can have
• 25:19
  Speaker 1: any energy, quantum mechanics is fine with that. Now you
• 25:22
  Speaker 1: take that same electron, you say, okay, you're now in
• 25:24
  Speaker 1: orbit around a hydrogen atom. Well, you still have to
• 25:27
  Speaker 1: obey the rules of quantum mechanics, and in this case,
• 25:29
  Speaker 1: quantum mechanics says are there's only certain solutions to the
• 25:32
  Speaker 1: math here. Only certain energies of the electron make the
• 25:36
  Speaker 1: math work. The wave function of the electron has to
• 25:39
  Speaker 1: satisfy some conditions, and that's only true for a certain
• 25:43
  Speaker 1: values of the electron energy. So you can't have an
• 25:45
  Speaker 1: electron with an arbitrary energy around a proton. There's a
• 25:49
  Speaker 1: ladder of values there, and that determines whether the electron
• 25:52
  Speaker 1: can absorb the energy of a passing photon.
• 25:55
  Speaker 3: I guess it's sort of like, you know, the Earth
• 25:57
  Speaker 3: is going around the Sun in an orbit, but the
• 25:59
  Speaker 3: Earth is it's not restricted to what that orbit can be.
• 26:02
  Speaker 3: Like if a meteor hits Earth within a force, it
• 26:05
  Speaker 3: is going to speed us up or slow us down,
• 26:07
  Speaker 3: and it's going to change the path of our orbit.
• 26:10
  Speaker 3: But you're saying sort of like in an electron around
• 26:12
  Speaker 3: an atom, it's not like it can be in any orbit.
• 26:15
  Speaker 3: It can only be in like certain slots of that orbit.
• 26:18
  Speaker 3: Like it can orbit here or over there, or in
• 26:21
  Speaker 3: this circle or in that circle. I know it's not
• 26:23
  Speaker 3: really a circle, but it's sort of that. I mean,
• 26:25
  Speaker 3: it can only circle around the nuclei or a certain grooves, right.
• 26:30
  Speaker 1: Yeah, that's a great contrast because the example of Earth
• 26:33
  Speaker 1: is a classical example. There's no quantum mechanics there. We're
• 26:36
  Speaker 1: talking about gravity, which is a classical theory, and there's
• 26:39
  Speaker 1: an infinite number of possible solutions for an orbit. If
• 26:42
  Speaker 1: you pick a radius for the Earth's orbit, I can
• 26:44
  Speaker 1: tell you exactly what velocity it has to have in
• 26:47
  Speaker 1: order to have that orbit. So there's an infinite number
• 26:49
  Speaker 1: of possible orbits there. In the case of the electrons,
• 26:52
  Speaker 1: it's really very different mathematics that determines whether the electron
• 26:56
  Speaker 1: can be in a particular state or not. You said,
• 26:58
  Speaker 1: it's not really in an orbit, it's in a quantum state,
• 27:01
  Speaker 1: which means it's satisfying a different equation. In this case,
• 27:03
  Speaker 1: it's Schrodinger's equation, which is a quantum equation of the
• 27:06
  Speaker 1: wave function, and that wave function has periodicity to it,
• 27:10
  Speaker 1: so the wave function basically has to wrap itself around
• 27:12
  Speaker 1: the atom in a way that builds upon itself. It
• 27:15
  Speaker 1: doesn't cancel itself out, So you can fit in like
• 27:17
  Speaker 1: an integer number of half wavelengths of this wave function,
• 27:21
  Speaker 1: so that things like support each other. You get like
• 27:23
  Speaker 1: a standing wave solution effectively instead of things like canceling
• 27:27
  Speaker 1: themselves out. Just the same way that like on a
• 27:29
  Speaker 1: guitar string, you can have a certain number of modes
• 27:32
  Speaker 1: of a guitar string, right, it can oscillate the whole string,
• 27:35
  Speaker 1: or you can have a node in the middle so
• 27:36
  Speaker 1: both halves are oscillating, or you can have two nodes,
• 27:39
  Speaker 1: or you get like three little oscillating pieces. In the
• 27:42
  Speaker 1: same way, the electron has to satisfy a wave equation,
• 27:45
  Speaker 1: not a gravitational equation, and that's where the energy levels
• 27:48
  Speaker 1: come from comes from confining it to being around the atom,
• 27:52
  Speaker 1: which changes the solutions to the equation.
• 27:54
  Speaker 3: Okay, so now I have an electron orbiting around a nuclei.
• 27:58
  Speaker 3: It's a wave function, it's a quantum object. It's sort
• 28:01
  Speaker 3: of like snaps into a certain wave shape around the
• 28:04
  Speaker 3: nuclei and you're saying that a photon hits it, and
• 28:06
  Speaker 3: the electron is like nope, I like where I am now?
• 28:10
  Speaker 3: No thanks, or that's not enough to get me to
• 28:12
  Speaker 3: the next step in the ladder. I'm just gonna totally
• 28:15
  Speaker 3: ignore you. Is that what's happening.
• 28:17
  Speaker 1: That's exactly what's happening. If a photon comes along and
• 28:20
  Speaker 1: it has enough energy to bump the electron to the
• 28:22
  Speaker 1: next stage, it gets absorbed. If it has too much
• 28:25
  Speaker 1: energy to get the electron to the next stage and
• 28:27
  Speaker 1: not enough to get it like two steps up, then
• 28:30
  Speaker 1: it gets ignored. Right, So it gets absorbed if it
• 28:33
  Speaker 1: has the right energy to move the electron up one
• 28:36
  Speaker 1: or two or seven some integer number of levels, and
• 28:39
  Speaker 1: it gets ignored. If the electron would not have a
• 28:42
  Speaker 1: solution anymore if it absorbed this photon, then it just
• 28:45
  Speaker 1: doesn't happen. Wait.
• 28:47
  Speaker 3: Wait, wait, So, like if an electron is going around
• 28:49
  Speaker 3: a nuclei, it's in a cloud, and it gets one
• 28:52
  Speaker 3: and a half as much energy from a photon that
• 28:55
  Speaker 3: it needs to get to the next level, it's not
• 28:57
  Speaker 3: going to take that one and then throw away the remainder.
• 29:00
  Speaker 3: It's just totally gonna ignore the whole thing.
• 29:02
  Speaker 1: Just totally gonna ignore the whole thing. It can absorb
• 29:05
  Speaker 1: something that has two steps and then emit one, right,
• 29:08
  Speaker 1: or it can absorb something that has like seven and
• 29:10
  Speaker 1: emit four photons, But it has to be on that ladder.
• 29:14
  Speaker 3: How exact does it need to be, like exactly to
• 29:16
  Speaker 3: the one infinite decimal, you know what I mean? Like,
• 29:19
  Speaker 3: where are the chances that the photon will have the
• 29:22
  Speaker 3: exact amount of energy needs or does it just need
• 29:25
  Speaker 3: to be around the same energy.
• 29:27
  Speaker 1: Well, there's always uncertainty in quantum mechanics, right, So every
• 29:30
  Speaker 1: photon has an uncertain amount of energy. You can never
• 29:32
  Speaker 1: measure it precisely.
• 29:33
  Speaker 3: Is that true? Really? I thought they had like specific frequencies.
• 29:37
  Speaker 1: Well, a photon is created by a quantum process, which
• 29:39
  Speaker 1: usually means that there is some uncertainty there. Right, there's
• 29:42
  Speaker 1: always a little bit of fuzz in all of these
• 29:44
  Speaker 1: processes which allow these things to overlap. The energy levels
• 29:47
  Speaker 1: that we're talking about come from a simplified model of
• 29:50
  Speaker 1: the nucleus, right, and in reality these things are a
• 29:53
  Speaker 1: little bit fuzzier.
• 29:54
  Speaker 7: Right.
• 29:54
  Speaker 1: The physics is a little bit more complicated. There's other interaction,
• 29:57
  Speaker 1: so there's always a little bit of fuzz on these
• 30:00
  Speaker 1: energy levels. And the atom that produced the photon on
• 30:03
  Speaker 1: the other side of the universe or whatever, may have
• 30:05
  Speaker 1: produced it at a certain energy level or a little
• 30:07
  Speaker 1: bit higher, a little bit lower, So there's enough fuzz
• 30:09
  Speaker 1: and quantum mechanics to mean that there's a non zero
• 30:12
  Speaker 1: probability for the photon to have the right energy to
• 30:14
  Speaker 1: be absorbed by the electron.
• 30:16
  Speaker 3: Hmmm. I guess when they interact, then the wave functions
• 30:20
  Speaker 3: collapse and then you figure out if it has the
• 30:22
  Speaker 3: right amount. But it seems very unlikely they would have
• 30:24
  Speaker 3: the exact same amount.
• 30:25
  Speaker 1: Yeah, that's where the fuzz comes in. So you get
• 30:27
  Speaker 1: a little bit of width to these things, so you
• 30:29
  Speaker 1: have a probability for them to overlap. It's not like
• 30:32
  Speaker 1: you're throwing a dart in an infinitely sized board and
• 30:34
  Speaker 1: having to hit exactly the right spot.
• 30:36
  Speaker 3: I see. It's like you have a fuzzy dart and
• 30:38
  Speaker 3: the target is fuzzy too, and as long as you
• 30:40
  Speaker 3: sort of get it in the rhine ballpark, then it's
• 30:43
  Speaker 3: going to knock that electron or not.
• 30:45
  Speaker 1: Yeah, exactly. And that explains a lot of atomic physics, right.
• 30:48
  Speaker 1: That explains why certain gases look certain colors. That explains
• 30:52
  Speaker 1: why when you have a fire you might get like
• 30:54
  Speaker 1: green or blue flashes in it. Or if you did
• 30:56
  Speaker 1: that experiment in high school chemistry where you put copper
• 30:59
  Speaker 1: in your bunsen and it glows green, explains a lot
• 31:02
  Speaker 1: of atomic physics because different kinds of materials have different
• 31:05
  Speaker 1: energy levels, so they glow with different frequency photons and
• 31:08
  Speaker 1: they can absorb different frequency photons. And that's really cool
• 31:12
  Speaker 1: because it means we can tell what's in distant stars
• 31:15
  Speaker 1: because we can look at the spectrum of energy that
• 31:17
  Speaker 1: they emit. We can say, oh, look, these things are
• 31:20
  Speaker 1: emitting photons from the energy level that only comes from copper,
• 31:23
  Speaker 1: so we can tell there's copper in that star. Sometimes
• 31:26
  Speaker 1: these things appear as spikes in the spectrum. Sometimes they
• 31:28
  Speaker 1: appear as dips in the spectrum because like the atmosphere
• 31:31
  Speaker 1: of the star is absorbing those photons. But the point
• 31:34
  Speaker 1: is that there are energy levels to the atom, and
• 31:36
  Speaker 1: those determine whether the photon can interact with the electrons
• 31:39
  Speaker 1: around the atom or whether it gets ignored.
• 31:41
  Speaker 3: All right, So then we're talking about transparency. And so
• 31:44
  Speaker 3: if I shoot a photon at an atom, it's gonna
• 31:47
  Speaker 3: get up to the electron cloud there and it's going
• 31:49
  Speaker 3: to be like, no, I'm not the right energy. I'm
• 31:52
  Speaker 3: just going to keep going. Or is it the case
• 31:54
  Speaker 3: that they do interact? But then the end result is
• 31:57
  Speaker 3: the same and it just spits out a photon of
• 32:00
  Speaker 3: the same energy in the same direction.
• 32:02
  Speaker 1: Now, if they're the wrong energy, they just do not interact.
• 32:05
  Speaker 1: If they're the right energy, it gets absorbed and then
• 32:07
  Speaker 1: it can get re emitted. And that's a whole complicated
• 32:09
  Speaker 1: phenomena about reflection and refraction and all sorts of stuff.
• 32:12
  Speaker 1: In this case, for transparency, it's more about whether there's
• 32:15
  Speaker 1: an interaction. If it has the wrong energy levels, it
• 32:17
  Speaker 1: just doesn't interact. But that's the case for a single atom,
• 32:20
  Speaker 1: which is not really what's going on when you're looking
• 32:23
  Speaker 1: at light going through glass or when you're wondering why
• 32:25
  Speaker 1: light doesn't go through metal. It's much more complicated because
• 32:29
  Speaker 1: now you're packing a lot of atoms together, and so
• 32:32
  Speaker 1: the rules about what happens to those electrons now change.
• 32:35
  Speaker 3: M let's dig into that. What's going on there.
• 32:38
  Speaker 1: So remember the picture we were talking about earlier. When
• 32:41
  Speaker 1: a photon is approaching like a sheet of metal or
• 32:43
  Speaker 1: a sheet of iron or a sheet of marble or something,
• 32:46
  Speaker 1: it's facing a whole wall of atoms, not individual atoms.
• 32:50
  Speaker 1: We talked about the energy levels of an individual atom.
• 32:52
  Speaker 1: But when you bring these things together to make a grid,
• 32:55
  Speaker 1: then the atoms bond They're not just like near each other,
• 32:58
  Speaker 1: they really are bonded. And if you remember your high
• 33:01
  Speaker 1: school chemistry, that means that they are sharing electrons. Sometimes
• 33:04
  Speaker 1: the electron will like be around one nucleus, sometimes around
• 33:07
  Speaker 1: another nucleus. So from the point of view of the electron,
• 33:10
  Speaker 1: what happens is that you no longer really belonging to
• 33:13
  Speaker 1: one nucleus. Now you can think of like the whole
• 33:16
  Speaker 1: grid of nuclei as having a bunch of energy levels
• 33:20
  Speaker 1: for the electrons. Some of the inner electrons are trapped
• 33:23
  Speaker 1: around nucleus, but the outer electrons can flow between them,
• 33:26
  Speaker 1: and that creates a whole complicated set of energy levels.
• 33:29
  Speaker 1: And instead of having these very specific ladders, now you
• 33:32
  Speaker 1: have this like spectrum energy levels. The electrons have lots
• 33:36
  Speaker 1: more options of the energy levels they can be at.
• 33:39
  Speaker 3: I guess I wonder if it's sort of like, you know,
• 33:41
  Speaker 3: we're orbiting around the Sun and we're sort of stuck
• 33:44
  Speaker 3: in this orbit, but if another solar system came pretty close,
• 33:48
  Speaker 3: maybe Jupiter might be like, oh, sometimes it might do
• 33:51
  Speaker 3: like a little figure eight and sometimes leave our solar
• 33:54
  Speaker 3: system and go take a loop around that other Sun
• 33:56
  Speaker 3: and then come back and is that sort of what's
• 33:58
  Speaker 3: happening to the electrons.
• 33:59
  Speaker 1: Yeah, that's exactly what's happening to the electrons. There's lots
• 34:02
  Speaker 1: more options for them. They don't have to just stick
• 34:04
  Speaker 1: around one nucleus. They interact with lots of different nuclei.
• 34:07
  Speaker 1: So that has the consequence of sort of spreading these
• 34:11
  Speaker 1: sharper atomic orbitals and making them even fuzzier. So instead
• 34:14
  Speaker 1: of even really thinking about energy levels now, physicists talk
• 34:18
  Speaker 1: about the possibilities for electrons in these materials as energy bands,
• 34:22
  Speaker 1: and you may have heard of like the valence band
• 34:25
  Speaker 1: or the conduction band. These are like a spectra of
• 34:27
  Speaker 1: energy levels available to the electron. Instead of being more
• 34:31
  Speaker 1: like a ladder, they get blurred together, so there's lots
• 34:33
  Speaker 1: of really really fine steps. It's still technically a ladder,
• 34:37
  Speaker 1: but there's many many more steps there.
• 34:39
  Speaker 3: Yeah, I guess it's sort of like in one atom,
• 34:41
  Speaker 3: the electron's stuck in one particular rut or groove or orbit.
• 34:45
  Speaker 3: That's one extreme. The electron is a free floating electron
• 34:48
  Speaker 3: adem sit in space by itself. When atoms are sort
• 34:51
  Speaker 3: of bonded together in the material, you're saying the electrons
• 34:54
  Speaker 3: are sort of in the middle, like they're not quite
• 34:56
  Speaker 3: stuck to one particular atom, but they're not quite free either,
• 35:00
  Speaker 3: and so they have limited options. But they don't have
• 35:02
  Speaker 3: just one option, and so there's sort of a range
• 35:04
  Speaker 3: of photons they can absorb.
• 35:06
  Speaker 1: Yeah, exactly. And it depends a little bit on the
• 35:08
  Speaker 1: temperature of the object. If the object is really really
• 35:10
  Speaker 1: cold and the electrons don't have a lot of energy,
• 35:13
  Speaker 1: then they've all like settled down to their minimum energy
• 35:16
  Speaker 1: and mostly they are orbiting an individual nuclei and they're
• 35:20
  Speaker 1: mostly stuck, and so the electrons don't flow very much.
• 35:22
  Speaker 1: If the thing is hot, then a lot of the
• 35:24
  Speaker 1: electrons have more energy. They have enough energy to like
• 35:27
  Speaker 1: hop from nuclei to nuclei, and so they can flow
• 35:30
  Speaker 1: a little bit better.
• 35:31
  Speaker 3: Wait, what so that if I heat something up or
• 35:33
  Speaker 3: cool it down, I can make it go transparent or
• 35:36
  Speaker 3: not transparent.
• 35:36
  Speaker 1: No, by heating it up, you're not changing the energy
• 35:39
  Speaker 1: levels that are available. You're just changing where the electrons are.
• 35:42
  Speaker 1: Like instead of all being in the lowest energy levels,
• 35:45
  Speaker 1: now they're in higher energy levels. I'm just talking about
• 35:47
  Speaker 1: which energy levels are filled up. In some cases, the
• 35:50
  Speaker 1: electrons are sort of stuck when this stuff is cold,
• 35:52
  Speaker 1: the electrons fill the lower energy levels and they're more
• 35:54
  Speaker 1: stuck to the nuclei, and when the object is warmer,
• 35:57
  Speaker 1: they sort of jump out of those and they're freer
• 35:59
  Speaker 1: to move around from nucleus to nucleus.
• 36:01
  Speaker 3: Cool. Well, let's get a little bit deeper into the
• 36:05
  Speaker 3: material and see what happens when photons of different frequencies
• 36:09
  Speaker 3: try to go through it, and what it all means
• 36:12
  Speaker 3: about transparency in the universe. But first, let's take another
• 36:16
  Speaker 3: quick break. All right, we're talking about transparency, and something
• 36:31
  Speaker 3: that comes to mind is, I don't know if you
• 36:34
  Speaker 3: read old comic books, or you read comic books when
• 36:36
  Speaker 3: you were a kid, there was always an ad in
• 36:38
  Speaker 3: the bag for like X ray glasses, and I always wondered,
• 36:41
  Speaker 3: like are those for real? Or like how can they
• 36:45
  Speaker 3: sell something so bogus out of what's going on? I
• 36:47
  Speaker 3: always wanted to order one, but I couldn't because I
• 36:49
  Speaker 3: wasn't in Panama. Do you know what I'm talking about?
• 36:51
  Speaker 3: Do you know what they were actually selling?
• 36:53
  Speaker 1: Do know those ads? And I also wanted those and
• 36:56
  Speaker 1: I wanted them to be real, but I also never
• 36:58
  Speaker 1: bought them because I was pretty sure they were bogus.
• 37:02
  Speaker 1: I mean, you can see through things with X rays,
• 37:04
  Speaker 1: and we'll talk about why that happens, why high energy
• 37:08
  Speaker 1: photons from X rays can pass through materials sometimes when
• 37:12
  Speaker 1: lower energy photons can't. But those glasses can't let you
• 37:15
  Speaker 1: see X rays, and they definitely don't generate X rays, right,
• 37:18
  Speaker 1: You're not shooting X rays through stuff. So I'm pretty
• 37:22
  Speaker 1: sure it was totally bogus.
• 37:24
  Speaker 3: Or maybe not. I don't know. We can't say for sure.
• 37:27
  Speaker 1: Well, folks out there, if you bought those X ray
• 37:29
  Speaker 1: glasses and they did let you see through things, please
• 37:32
  Speaker 1: write to us and let us hear.
• 37:34
  Speaker 3: Those people are probably rich from a you know, stealing
• 37:37
  Speaker 3: a bank vaults and things like that. All right, So
• 37:40
  Speaker 3: we're talking about transparency. And you know, when you put
• 37:43
  Speaker 3: a bunch of adoms together in the material, they form
• 37:45
  Speaker 3: this kind of extended fuzzy cloud of electrons that might
• 37:49
  Speaker 3: blocklide or not. And so whether a photon gets through
• 37:52
  Speaker 3: that depends on its energy. If it has the energy
• 37:55
  Speaker 3: that the electrons in that material like, then it gets
• 37:58
  Speaker 3: it's going to get absorbed right and not go through exactly.
• 38:01
  Speaker 1: So the basic picture is the same photon approaches this
• 38:05
  Speaker 1: now grid of atoms and if it finds an electron
• 38:08
  Speaker 1: that can accept its energy, if the electron can go
• 38:11
  Speaker 1: from its current quantum state to an allowed quantum state,
• 38:14
  Speaker 1: it will absorb that photon. But the picture of the
• 38:17
  Speaker 1: energy levels is different from a single atom than with
• 38:20
  Speaker 1: the grid of atoms, and the single atom you had
• 38:21
  Speaker 1: the ladder, there was sort of sharper energy levels in
• 38:24
  Speaker 1: the grid of atoms. Now you have these bands of
• 38:27
  Speaker 1: energy levels, and you might think, oh, that makes it
• 38:29
  Speaker 1: possible for the electron to absorb basically any photon. It's
• 38:32
  Speaker 1: a little bit more complicated than that because we discovered
• 38:35
  Speaker 1: that there are these gaps in the energy levels. It's
• 38:37
  Speaker 1: not like any possible energy level is allowed for an
• 38:40
  Speaker 1: electron in these materials, the way it is for an
• 38:42
  Speaker 1: electron in free space. There are still electron energy levels
• 38:45
  Speaker 1: that are not allowed. So there's this band of electron
• 38:49
  Speaker 1: energies called the valiance band, where the electrons mostly hang
• 38:52
  Speaker 1: out in a random material, and then there's a band
• 38:54
  Speaker 1: of energies called the conduction band, where electrons can move
• 38:58
  Speaker 1: around really freely from atom to adm them and there's
• 39:00
  Speaker 1: sometimes a gap between them where electrons can't be and
• 39:04
  Speaker 1: that can prevent electrons from absorbing energy of a passing photon.
• 39:08
  Speaker 3: M you're saying, like, you can have a material that
• 39:11
  Speaker 3: accepts or lets through lights of a certain range of frequencies,
• 39:16
  Speaker 3: then it doesn't let them through, and then it does
• 39:18
  Speaker 3: for a different range of frequencies, it does lead light.
• 39:21
  Speaker 1: Through exactly just like with the atom. It can absorb
• 39:24
  Speaker 1: some frequencies and not other frequencies for a grid of atoms.
• 39:27
  Speaker 1: For a whole solid material, it can absorb some frequencies
• 39:31
  Speaker 1: frequencies where it can hit the electron and jump it
• 39:34
  Speaker 1: over this gap between the bands, and it can't absorb
• 39:37
  Speaker 1: photons of other frequencies, photons that don't have enough energy
• 39:41
  Speaker 1: to get the electrons from one band to another. And
• 39:44
  Speaker 1: so different kind of materials have a different sized gap
• 39:47
  Speaker 1: between these bands, and so in solid state physics they
• 39:50
  Speaker 1: call this the band gap, right, the gap between the
• 39:52
  Speaker 1: typical energy levels of the electron and the conduction band
• 39:56
  Speaker 1: where electrons are good at like flowing, and some kind
• 39:58
  Speaker 1: of materials like meta have a very very small band gap.
• 40:02
  Speaker 1: The conduction band is basically right on top of the
• 40:05
  Speaker 1: valiance band. There's basically no gap there, and so electrons
• 40:08
  Speaker 1: are very good at absorbing photons of a huge range
• 40:11
  Speaker 1: of energies because there's a huge spectrum there and other
• 40:14
  Speaker 1: materials is a big gap. And in order for an
• 40:16
  Speaker 1: electron to absorb a photon, it has to have enough energy,
• 40:20
  Speaker 1: and lots of photons just don't have enough energy, and
• 40:22
  Speaker 1: so the photons would pass right through the material without interacting.
• 40:27
  Speaker 3: Now, when you're talking about light and energy, the light
• 40:29
  Speaker 3: of a particular photon is related to its frequency, right
• 40:33
  Speaker 3: mostly to it almost or everything to its frequency, and
• 40:36
  Speaker 3: so you're really talking about its color, right.
• 40:38
  Speaker 1: Yeah, exactly. The energy of a photon doesn't relate to
• 40:40
  Speaker 1: its speed. Right. When we think about the energy of
• 40:42
  Speaker 1: an electron, we think about its speed, but photons are
• 40:45
  Speaker 1: all moving at the same speed. The thing that differentiates
• 40:48
  Speaker 1: a high and low energy electron is its frequency, how
• 40:52
  Speaker 1: fast the electromagnetic fields are wiggling. And as you say
• 40:55
  Speaker 1: that frequency we interpret as color. The photons themselves don't
• 40:59
  Speaker 1: have color. It's not like a photon is a red
• 41:01
  Speaker 1: photon or a green photon just has a certain frequency
• 41:05
  Speaker 1: when it hits our eyeballs our brains give us the
• 41:07
  Speaker 1: experience of red or green or blue or whatever, and
• 41:10
  Speaker 1: that's a whole philosophical question. But yeah, we associate colors
• 41:13
  Speaker 1: with certain frequencies.
• 41:14
  Speaker 3: Yes, but we don't talk about philosophy here. But like that,
• 41:18
  Speaker 3: if a photon has a certain frequency, it is a
• 41:21
  Speaker 3: red photon, right, Like to our eyes it would read
• 41:24
  Speaker 3: as red.
• 41:24
  Speaker 1: It would read as red. Yeah, And some photons are
• 41:27
  Speaker 1: above the visible spectrum, and so we say they're X
• 41:29
  Speaker 1: ray photons, or their gamma rays, or their UV photons, right,
• 41:34
  Speaker 1: So we can give names to the different parts of
• 41:36
  Speaker 1: the frequency spectrum. Some of them we give them colors,
• 41:39
  Speaker 1: some of them we just give them labels. Radio waves,
• 41:41
  Speaker 1: for example, our photons a very very long frequency, well
• 41:46
  Speaker 1: below what we can see, even below the infrared.
• 41:49
  Speaker 3: Right, And so that determines whether or not a materials
• 41:52
  Speaker 3: transparent to different kinds of light. Because X rays sort
• 41:55
  Speaker 3: of lead you see through your body and your bones, right.
• 41:59
  Speaker 3: That's because they have a high energy and the electrons
• 42:02
  Speaker 3: in your body can absorb them, so they sort of
• 42:04
  Speaker 3: go through.
• 42:04
  Speaker 1: Yeah. Really interestingly, X rays can pass through the soft
• 42:08
  Speaker 1: tissues of your body, but they can't pass through your bones,
• 42:12
  Speaker 1: which is why when you see an X ray, what
• 42:14
  Speaker 1: you're looking at is basically only the bones. Because that's
• 42:16
  Speaker 1: the thing that the X rays didn't pass through, so
• 42:19
  Speaker 1: it passes through everything else. Your body is transparent to
• 42:22
  Speaker 1: X rays except for your bones. That's why you can
• 42:25
  Speaker 1: tell the difference between the bones and the not bones
• 42:27
  Speaker 1: part on the X ray.
• 42:29
  Speaker 3: Now is that because the I guess bones are made
• 42:31
  Speaker 3: out of the different material than my muscles, and so
• 42:35
  Speaker 3: my muscles don't absorb X rays what my bones do
• 42:38
  Speaker 3: because of the you know, the bonds between the atoms exactly.
• 42:44
  Speaker 1: It's the band gap of the material that determines whether
• 42:47
  Speaker 1: or not you can absorb photons of a specific frequency. So,
• 42:51
  Speaker 1: for example, in a conductor like a metal, like a
• 42:53
  Speaker 1: sheet of steel, the band gap is really really small.
• 42:56
  Speaker 1: It's very easy to get an electron up into that
• 42:58
  Speaker 1: conduction band where it can low around. And that's why
• 43:01
  Speaker 1: these things conduct electricity very easily because it's easy to
• 43:04
  Speaker 1: have electrons that slide around in the material. So conductor
• 43:07
  Speaker 1: like a metal, right, it's really easy to get those
• 43:09
  Speaker 1: electrons flying around. It also means it's easy to absorb
• 43:13
  Speaker 1: that energy. So that's why things like metals and conductors
• 43:16
  Speaker 1: are good at conducting electricity and good at absorbing photons
• 43:20
  Speaker 1: and bad at being see through. So that's why a
• 43:22
  Speaker 1: sheet of metal, for example, is not.
• 43:24
  Speaker 3: Transparent unless it's I guess a wire mesh.
• 43:29
  Speaker 1: Yeah, exactly, unless it's a screen, which is why if
• 43:32
  Speaker 1: you remember like Star Trek and they had like transparent
• 43:34
  Speaker 1: aluminium in Star Trek four or whatever, I always like, well,
• 43:37
  Speaker 1: you can't really do that. That is not something we
• 43:40
  Speaker 1: know how to do. Although you know, far future societies
• 43:42
  Speaker 1: maybe they figured it out.
• 43:43
  Speaker 3: I'm not sure I'm familiar with that level of trivia
• 43:46
  Speaker 3: for the Story Trek movies.
• 43:47
  Speaker 5: But I.
• 43:49
  Speaker 1: Remember they got the whales and they had to build
• 43:52
  Speaker 1: an aquarium for the whales, and how are they going
• 43:53
  Speaker 1: to hold all this water?
• 43:54
  Speaker 3: I remember the whales? Yeah, right, but I guess what
• 43:57
  Speaker 3: do you mean though, Like aluminium is not it's transparent
• 44:00
  Speaker 3: to visible light, but it is still transparent to other
• 44:03
  Speaker 3: kinds of light, right, like X rays sort of go
• 44:05
  Speaker 3: through metal. No, or do metals like block all light.
• 44:08
  Speaker 1: It's always the case that it depends on the frequency, right,
• 44:11
  Speaker 1: and so you have to have the right frequency to
• 44:14
  Speaker 1: match the energy levels that the object can absorb. If
• 44:17
  Speaker 1: you have a huge amount of energy, then probably you're
• 44:20
  Speaker 1: going to knock the electrons out of the material. Right,
• 44:23
  Speaker 1: then we're getting into the case of like the photoelectric effect.
• 44:26
  Speaker 1: So you zap like gamma rays against aluminum, then there's
• 44:30
  Speaker 1: definitely going to be an interaction there, but it's going
• 44:32
  Speaker 1: to knock the whole electron out of the material. It's
• 44:34
  Speaker 1: not just going to push it up to some energy level.
• 44:37
  Speaker 1: So at some point this picture breaks down.
• 44:39
  Speaker 3: Oh what So at some point you have enough energy
• 44:42
  Speaker 3: where the electron JA doesn't someone to say in any groove,
• 44:45
  Speaker 3: it just flies out into space.
• 44:46
  Speaker 1: Really, yeah, exactly, you can shine light on metal and
• 44:49
  Speaker 1: boil off electrons if you have enough energy. It's like
• 44:53
  Speaker 1: a highest level band and above that then electrons are
• 44:56
  Speaker 1: just free. Again, you've like broken it out of physics jail.
• 45:00
  Speaker 3: You get to pay two hundred bucks. Though exactly, Now,
• 45:04
  Speaker 3: what happens on the other spectrum, Like what if a
• 45:06
  Speaker 3: photon has too little energy like a super infrared or
• 45:10
  Speaker 3: something like that, or radio wave as you said, that
• 45:13
  Speaker 3: still goes through metal and other things.
• 45:15
  Speaker 1: Right, No, radio waves do not go through metal. Right.
• 45:18
  Speaker 1: That's why, for example, your phone call is dropped if
• 45:20
  Speaker 1: you're in an elevator because metal is like a Faraday cage.
• 45:24
  Speaker 1: It will block radio waves even classically, right, the electrons
• 45:28
  Speaker 1: in the material will reorganize themselves to cancel out an
• 45:31
  Speaker 1: electric field. But from a quantum mechanical point of view,
• 45:33
  Speaker 1: a conductor can absorb very very low energy photons because
• 45:36
  Speaker 1: the gap there is very small, and so it can
• 45:39
  Speaker 1: absorb very very low energy photons.
• 45:41
  Speaker 3: But I guess what's going on there though, Like, if
• 45:44
  Speaker 3: it's just a single atom and I have an electron orbiting,
• 45:46
  Speaker 3: if the forodan has very little energy, I'm going to
• 45:50
  Speaker 3: ignore it too, are I?
• 45:51
  Speaker 1: Yeah, absolutely you are. In the case of an individual atom,
• 45:54
  Speaker 1: then there are photons that have too low an energy
• 45:57
  Speaker 1: to move the electron up from level one, level two,
• 46:00
  Speaker 1: or level seven to level eight exactly. That can happen
• 46:03
  Speaker 1: in a solid. Now you have a whole spectrum of
• 46:06
  Speaker 1: energy levels, and so there's lots of very very fine
• 46:09
  Speaker 1: gradations allowed there. So materials can absorb low energy electrons
• 46:14
  Speaker 1: because there's a very very fine mesh of energy levels.
• 46:16
  Speaker 3: Is there a bottom limit there? Like, is there an
• 46:19
  Speaker 3: energy for my photon for which it's even outside of
• 46:23
  Speaker 3: the gap of material with lots of electrons.
• 46:26
  Speaker 1: There might be a lower limit there. I mean, even
• 46:28
  Speaker 1: conductors do have some kind of a band gap, so
• 46:32
  Speaker 1: you might need a minimum energy to get them up
• 46:35
  Speaker 1: from the valiance band to the conduction band, and there
• 46:38
  Speaker 1: might even be a limit within those bands a minimum energy,
• 46:41
  Speaker 1: so the other might be a limit. Very very low
• 46:44
  Speaker 1: energy photons could be ignored even by conductors. But the
• 46:47
  Speaker 1: other side of the coin are materials like insulators. Take glass,
• 46:50
  Speaker 1: for example. Glass is not a conductor because it has
• 46:54
  Speaker 1: a large gap between these energy levels. So mostly electrons
• 46:57
  Speaker 1: in the glass are not free to move around. They're
• 46:58
  Speaker 1: mostly stuck to the atom that they are around, and
• 47:02
  Speaker 1: there's a band gap there. If you want to push
• 47:03
  Speaker 1: an electron up to the next energy level, there's like
• 47:06
  Speaker 1: a big gap between the energy levels that's normally in
• 47:09
  Speaker 1: and the first one that's available. It's like a few
• 47:12
  Speaker 1: electron bolts, and so photons that hit glass in the
• 47:15
  Speaker 1: visible spectrum mostly do not have enough energy to get
• 47:19
  Speaker 1: the electron up to the conduction band. And so that's
• 47:22
  Speaker 1: why visible light photons do pass through glass. They pass
• 47:27
  Speaker 1: right through this whole grid of atoms and all those electrons,
• 47:30
  Speaker 1: but they don't have enough energy to move the electrons
• 47:32
  Speaker 1: up to the next band, and so they're ignored and
• 47:35
  Speaker 1: they pass right through.
• 47:36
  Speaker 3: You mean like a material like glass. It's like the
• 47:38
  Speaker 3: atoms are basically it's just a bunch of individual atoms
• 47:42
  Speaker 3: hanging out together. They're not sharing a lot of electrons,
• 47:45
  Speaker 3: which is what you need to make a good conductor.
• 47:47
  Speaker 3: They're mostly just doing what they would do normally on
• 47:50
  Speaker 3: their own, and so you have a very limited number
• 47:53
  Speaker 3: of frequencies that it blocks.
• 47:55
  Speaker 1: The picture is a little bit more complicated. I mean
• 47:57
  Speaker 1: the glass atoms still do interact with each other, so
• 47:59
  Speaker 1: they do this band of energy levels for the electrons
• 48:03
  Speaker 1: because they are bonded together, right, I mean, glass is
• 48:05
  Speaker 1: not a crystal, but still there are bonds between the
• 48:07
  Speaker 1: atoms they are interacting, so there is a spectrum of
• 48:10
  Speaker 1: energy levels the electrons can be in For a glass
• 48:13
  Speaker 1: that's not just like an atom. It's not just like
• 48:15
  Speaker 1: a sharp layer, but it's mostly a full band of electrons.
• 48:19
  Speaker 1: But that band is mostly filled. Then the electrons can't
• 48:22
  Speaker 1: really go anywhere. It's like if you're on a plane,
• 48:24
  Speaker 1: every seat is taken, then you can't like move from
• 48:27
  Speaker 1: seat to seat, and the next energy level above that
• 48:29
  Speaker 1: is kind of high. Right, you can't like get to
• 48:31
  Speaker 1: first class. You need a lot of energy to get
• 48:33
  Speaker 1: to first class in a glass, and so everybody's basically
• 48:37
  Speaker 1: stuck in their seat in coach, and the electrons can't
• 48:40
  Speaker 1: really absorb little amounts of energy. They need a lot
• 48:43
  Speaker 1: of energy to get promoted up to first class, which
• 48:46
  Speaker 1: is the next band of energy levels. In a glass
• 48:49
  Speaker 1: and a conductor, that band is much much lower, so
• 48:51
  Speaker 1: it doesn't take as much energy to get up there,
• 48:54
  Speaker 1: and glass the band is really large, it's really hard
• 48:56
  Speaker 1: to get promoted up to the next set of energy levels.
• 49:00
  Speaker 3: I think what you're saying is that glass is transparent
• 49:03
  Speaker 3: for a wider range of frequencies of light, which just
• 49:07
  Speaker 3: happened to be in our visible spectrum. But glass is opaque.
• 49:10
  Speaker 3: It is not transparent to certain frequencies of light.
• 49:14
  Speaker 1: That's exactly right. For example ultraviolet, right, ultraviolet is light
• 49:17
  Speaker 1: with higher energy. You can't see ultraviolet light. It's the
• 49:21
  Speaker 1: kind that's going to give you a sunburn or can
• 49:22
  Speaker 1: hurt your eyeballs. But it has more energy, and it
• 49:25
  Speaker 1: has enough energy to bump one of these electrons up
• 49:29
  Speaker 1: over this gap into the conduction band, and so glass
• 49:32
  Speaker 1: can absorb uv photons. That's why you don't get a
• 49:35
  Speaker 1: sunburn if you're sunbathing through glass, glass is like sunscreen.
• 49:39
  Speaker 3: Wait what I can just put a glass over me
• 49:42
  Speaker 3: and I'm going to give you sunburn? Is that a
• 49:45
  Speaker 3: solid medical advice there?
• 49:47
  Speaker 1: That is not solid medical advice?
• 49:48
  Speaker 3: And just to be transparent, we're non medical.
• 49:50
  Speaker 1: Doctors, right, that's right. But it does block some of
• 49:54
  Speaker 1: the UV, so it would reduce your sunburn. It still
• 49:56
  Speaker 1: totally advise you to wear sunscreen, but glass is not transparent,
• 50:00
  Speaker 1: and to UV, the same way is to visible light,
• 50:03
  Speaker 1: absorbs a lot more of the UV.
• 50:05
  Speaker 3: Is that kind of what's going on with sunscreens? Like lotion?
• 50:08
  Speaker 5: Right?
• 50:08
  Speaker 7: Right?
• 50:08
  Speaker 3: It has materials that absorb UV.
• 50:11
  Speaker 1: Right, Yeah, exactly, your sunscreen is just opaque to UV.
• 50:15
  Speaker 1: It has stuff in it that could accept those photons
• 50:18
  Speaker 1: and absorb it rather than letting those UV photons pass
• 50:20
  Speaker 1: into your body and then cause damage.
• 50:22
  Speaker 3: Okay, Now, what makes a piece of glass like a
• 50:25
  Speaker 3: piece of red glass or a piece of blue glass?
• 50:27
  Speaker 1: So that often is because of doping. You like change
• 50:31
  Speaker 1: the energy levels of the glass by adding impurities, and
• 50:35
  Speaker 1: so these other molecules change the band gap, so make
• 50:38
  Speaker 1: it possible for different kinds of photons to be absorbed.
• 50:41
  Speaker 3: And so I guess you sort of narrow the band gap, right,
• 50:45
  Speaker 3: like regular glass wide range of photon frequencies that it
• 50:49
  Speaker 3: lets through, but like blue glass I imagine, has a
• 50:52
  Speaker 3: narrow gap where it only lets through light that is
• 50:55
  Speaker 3: bluish for example.
• 50:56
  Speaker 1: That's right. So sometimes people add like aluminium oxide the glass,
• 50:59
  Speaker 1: and that makes glass pink or red because it absorbs
• 51:02
  Speaker 1: the green and the blue photons. And so I actually
• 51:05
  Speaker 1: got an email from a listener, Matt Cleveland, who says,
• 51:08
  Speaker 1: what is it about the photons of sunlight that costs
• 51:10
  Speaker 1: some objects to fade and lose their color? What is
• 51:14
  Speaker 1: it that's breaking down? Why do some objects lose their
• 51:16
  Speaker 1: color from this interaction with the sun's photons and others
• 51:19
  Speaker 1: do not?
• 51:20
  Speaker 3: That's an interesting question. Yeah, Like if you leave your
• 51:23
  Speaker 3: T shirt out in the sun, it's going to get faded, right,
• 51:25
  Speaker 3: It's going to get bleached. That's kind of why your
• 51:27
  Speaker 3: hair also gets bleached. A little bit of view. Stay
• 51:29
  Speaker 3: out in the sun a lot.
• 51:30
  Speaker 1: Yeah, exactly, And that's mostly the UV light, Right. These
• 51:33
  Speaker 1: chemicals absorb UV light, and the UV light has a
• 51:36
  Speaker 1: lot of energy, so sometimes it breaks down those chemicals. Right,
• 51:40
  Speaker 1: we talked about like photons hitting electrons and banging them
• 51:43
  Speaker 1: out of materials well. UV light. Sometimes these atoms can
• 51:47
  Speaker 1: absorb it, but it also damages the atoms the same
• 51:49
  Speaker 1: way like can damage things in your body, and so
• 51:52
  Speaker 1: chemicals in objects can sometimes break down when they absorb
• 51:56
  Speaker 1: UV light. And what you'll notice is that red stuff
• 51:58
  Speaker 1: is especially suceptible to this because they absorb more high energy,
• 52:03
  Speaker 1: more blue and more UV photons. So things that look
• 52:06
  Speaker 1: red are things that absorb in the blue spectrum and
• 52:09
  Speaker 1: therefore absorb more UV light and are likely to fade
• 52:12
  Speaker 1: more in sunlight than things that are.
• 52:15
  Speaker 3: Blue, because if they're red, then that means they're mostly
• 52:18
  Speaker 3: reflecting the red part, but they're absorbing the blue light.
• 52:21
  Speaker 1: Yeah, exactly. And UV is like super blue.
• 52:24
  Speaker 3: But sometimes materials get harder in the sun, right, Like
• 52:27
  Speaker 3: if you leave a piece of rubber or a rubber
• 52:29
  Speaker 3: band or your car tires, they get more brittle as
• 52:32
  Speaker 3: they stay out in the sun longer.
• 52:34
  Speaker 1: Yeah, that's a similar process. You're not changing their transparency,
• 52:37
  Speaker 1: but still the UV light is changing the chemical composition
• 52:40
  Speaker 1: because it's being absorbed and it's breaking down some of
• 52:42
  Speaker 1: the bonds, and it's changing the chemical nature of the substance.
• 52:45
  Speaker 3: So if you put your car inside of a glass house.
• 52:50
  Speaker 3: Then they'll stay the same color and the tires will
• 52:52
  Speaker 3: stay bouncy. Is that what you're saying?
• 52:54
  Speaker 1: Yeah? Or if you smear your car and sunscreen either one.
• 52:56
  Speaker 3: Oh, I guess you have to be careful. You know,
• 52:58
  Speaker 3: you know what to say about cars that live in
• 53:00
  Speaker 3: glass houses. All right, Well, I guess it's an interesting
• 53:05
  Speaker 3: look into a very familiar thing that is all around us, right,
• 53:08
  Speaker 3: Like the screen on your phone is made out of
• 53:10
  Speaker 3: transparent glass, and your windows, and every time you go
• 53:13
  Speaker 3: to the doctor or the dentists and they take X rays,
• 53:15
  Speaker 3: it's like physics going on.
• 53:16
  Speaker 1: Right, There is physics going on everywhere. What's amazing to
• 53:20
  Speaker 1: me is that sometimes we can even unravel this like
• 53:23
  Speaker 1: microphysical picture of what's happening. Do explain our everyday experience.
• 53:28
  Speaker 1: Why things are squishy, why things are hard, why things
• 53:30
  Speaker 1: conduct electricity, why things are see through. It all comes
• 53:34
  Speaker 1: down to what's happening at the atomic level or the
• 53:37
  Speaker 1: subatomic level, And incredibly, that's a story we can sometimes
• 53:40
  Speaker 1: understand and even explain to you.
• 53:43
  Speaker 3: Yes, it's almost like the universe is transparent to science,
• 53:48
  Speaker 3: or it's like scientists have X ray glasses.
• 53:50
  Speaker 1: Or maybe the UV photons of the universe are just
• 53:53
  Speaker 1: frying our brains.
• 53:55
  Speaker 3: Because your skull is made at a glass. What's going
• 53:58
  Speaker 3: on there?
• 53:59
  Speaker 1: I'm going to go put sunscreen on my brain, yeah.
• 54:02
  Speaker 3: Or a hat, you know. They physics have invented hats
• 54:05
  Speaker 3: also which helps with sun damage.
• 54:09
  Speaker 1: Quantum hats. We should sell those all right.
• 54:11
  Speaker 3: Well, we hope you enjoyed that. Thanks for joining us,
• 54:14
  Speaker 3: See you next time.
• 54:23
  Speaker 1: Thanks for listening, and remember that. Daniel and Jorge Explain
• 54:26
  Speaker 1: the Universe is a production of iHeartRadio. For more podcasts
• 54:30
  Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
• 54:35
  Speaker 1: you listen to your favorite shows.