The Science of Everything Podcast - Episode 32: Light and Optics

Episode Date: April 14, 2012

A discussion of the nature of light and its interactions with matter, including a journey through the history of our understanding of light from Newton’s corpuscular theory through early wave theory and up to the modern wave-particle duality understanding. Following this is an analysis of the various ways light interacts with matter, including absorption, reflection, transmission, refraction, dispersion, and transparency.

You're listening to The Science of Everything podcast, episode 32, light and optics. And I'm your host, James Fodor. So in this episode, we're going to look at light and optics. Specifically, we're going to look at the different theories of how light works, the particle and the wave theory, and then we'll sort of combine those together in the modern understanding of electromagnetism, wave particle duality, and all that sort of thing. Very interesting. Once we've got a basic understanding of light, we'll go through and look at some basic

behaviors of light, how light behaves under certain conditions, or, or in certain interactions with matter, including the behavior of light at media between two different types of materials when it hits the boundary between those. Behaviors such as absorption, reflection, and transmission. We'll also take a look at colour and its relationship to the wavelength of light, and we'll talk about refraction and dispersion to interesting properties of light or behaviors of light. So before we start, it's probably a good idea if you had a bit of a background in some of the

prior physics episodes before listening to this one. Episode you could look at include Episode 9, Matter and Molecules, episode 14 on Principles of Quantum Mechanics. And definitely you'd want to have the background of episode 24 vibrations and waves. That one's probably the most important. Either listen to those episodes

or at least have some background in those generic topics. You especially need to know things like wavelength and frequency and so on, otherwise those don't really make sense when I talk about them in this episode. All right then, let's get into it. First question, what is light? This is a question that sort of plagued philosophers and scientists or, I guess, proto-scientists for thousands of years, because light seems to be somewhat of a mysterious phenomenon in the way it behaves. You know, it's not exactly matter, but clearly it interacts with matter and so on, so what is it exactly? So historically, there were basically two different theories of light, the particle theory and the wave theory. So I'll look at each of those in turn. Particle theory of light, it was originally called the corpuscular theory of light, was set forward in its sort of most developed form by Sir Isaac Newton, the same guy who came up. with the laws of motion and the law of gravity and so on.

The basic idea is that light is made up of tiny discrete particles called corpuscules, which travel in a straight line with a finite velocity and also possess kinetic energy. So that explains why light interacts with matter, because they actually possess kinetic energy and they travel with a finite velocity. And you could model these corpuscules by using light rays, which is just sort of like a line of light that you can draw on paper and manipulate them to sort of understand how light's going to interact with mass. matter in given circumstances. And you could use this light ray model to understand for

non-like shadows and reflection and so on. So Newton's corpuscular theory had some success in explaining, for example, reflection and other behaviors of light. However, one big problem with Newton's theory, particle theory of light, is it could not explain refraction. Refraction that I mentioned in episode 24 about vibrations and waves is the apparent, well, not apparent, it's the changing direction of light or waves upon entering a new medium. This has been clearly observed, for example, if you put light into a prism, you shine light at an angle onto a prism or some other translucent optical medium, it will change direction. So that's a well-observed phenomenon that Newton's theory could not explain. So what he had to do to try and to fit that

observation in with his theory was to assume that there was some force that acted on the light ray when it entered and also then when it exited the different media, that is the different substance that it's travelling through. So according to Newton's theory, when the particles of light started, just when they moved from traveling through the air to traveling through the prism, there's a force acted on them so that it caused them to change direction. And then the same thing happened when they left the prism, except the force acted in the opposite direction, so they sort of changed back directions again. That was rather ad hoc, and it didn't really sort of fit with the rest of the theory, but at least it fit the observations. And in order to make that

work, in order to make these sort of forces that he introduced fit with the observations, he had to assume that light accelerated in denser media. So the force of called the forces caused light to accelerate in denser media, like the prison, for example, that's denser than air. So Newton's forces that was supposed to be acting upon the light would have to accelerate the light in denser media and then decelerate it once it got into the less dense media again. Now, at the time Newton came up with that theory, there was no way of testing it, there was no way of measuring the speed of light, and so there was no way of telling whether that model of those forces acting on light as they changed media was accurate. So meanwhile,

Well, another famous scientist called Christian Hoygens, also around the sort of late 17th century, worked out a wave theory of light, where he proposed that light was a series of waves, so, you know, waves with wavelengths and frequency and so on, in a medium called the luminiferous ether. So remember, a wave requires a medium to travel in. You know, you have sound waves, the medium is air,

for water waves, the medium is water, and so on. So for Christian Hoygens' light waves, His proposed medium was called the ether, or the luminiferous ether, which was supposed to be some sort of just generic substance that filled all of space, even outer space and the vacuums and so on. It was supposed to be everywhere, and this medium propagated light waves. Now, this wave theory had a big advantage over Newton's theory in that it could explain refraction, because remember, refraction's a natural consequence of wave behavior as you, when you change media, especially when you change from moving into a more or less dense media where the waves can travel a different space.

speeds, and if you don't know what I'm talking about, refer back to episode 24 in vibrations and waves. So, if light was a wave, then it's not surprising at all that it changes speed and therefore change its direction when it moves from one medium to another. In contrast to Newton's model, also, in Hoygens' model, in his wave model, it predicted that light would slow down in denser medium, whereas, remember, Newton predicted that light would speed up in denser mediums, because that's how we could get his sort of forces to work getting the light to change the direction in the way it was observed to. And so remember, I said that at the time these theories were put forward, there was no way of measuring the speed of light in the different media, so there was no way

of telling which one was true, or which one was a better model. The speed of light in materials could not be measured until about 200 years later in 1862. So both of these theories sort of stood side by side for a while, although I think Newton's theory was generally preferred, because Newton was right on so many other matters. But in 1862, it was shown that light indeed traveled more slowly in denser medium, so that is light's going to travel more slowly in a prism than it in air and more slowly in water than it does in air and so on. So the denser the medium, the slower light travels generally. And with that observation, that disproved Newton's corpuscular theory of light, because his forces that he had to introduce, he didn't really have

any evidence of these forces, he just made them up because they enabled his model to fit the data, to fit the observation of refraction. In order for that to work, it required that light travelled faster in denser media. In 1986, it was proved that the reverse was true, light traveled slower in denser media, and therefore Newton's corpuscular theory was essentially rejected. And also sort of around that time or a little bit earlier, a number of other pieces of evidence were built up to support the wave theory of light, apart from refraction. So the thing we were talking about with the speed of light changing, that's refraction. That's the change in the direction of light or of a wave as it passes from one medium to another. Another piece of evidence,

the supported wave theory was diffraction, which remember is the spreading out of waves as they and when they pass through small slits, this was observed and measured to occur with light. That is, when you pass light rays, or when you pass light beams through narrow slits, you actually observe a diffraction pattern with sort of light and dark shadows on a photographic border or another way of measuring the light at the end.

And this is a behavior of waves that you can't really explain with particles or on the copustular model. So diffraction is further sport of the wave model. Finally, Thomas Young in 1801 demonstrated a yet another effect that could really only be explained by the wave model of light, namely interference of different waves or different rays of light. And once again, interference is where you have essentially the peaks of one wave, sort of overlapping with the troughs of another wave and then

causing destructive or constructive interference, causing the wavelength to change when you combine multiple wavelengths together. Once again, refer back to the Vibrations and Waves podcast, if you don't know what I'm talking about here. This behavior of interference is only as only occurs with waves, it makes no sense with particles. And Thomas Young first observed this interference to occur in 1801, and so once again that's further support that light behaves as a wave. So sort of by the mid-late 19th century with interference, diffraction, and refraction, with the speed of light going slower, with light driving slower and denser media,

those three phenomena put together seemed to be strongly supportive of the wave theory of light. And so Newton's corpuscular theory sort of went out of fashion. It wasn't widely accepted anymore. The one big problem with the wave theory, though, it seemed to have all these successes and explained everything. The one big problem was that it required a medium for the light waves to propagate through, this so-called luminiferous ether.

But it was never really detected. It was just sort of a hypothetical. And so it was always a bit suspected, well, you know, we need to test for this ether. We need to see if we can actually detect it, because then that'll be like the Kudai Garah, in a sense, for the wave theory, if we can detect this ether that it travels through.

So a series of experiments were devised and devised and carried out in the late 19th century to try and detect the ether, and one very famous experiment, which I won't explain in detail, because it's sort of complicated. I may have explained it a little bit in a previous episode of quantum mechanics, actually, I can't remember, but it's called the Michaelson-Morley experiment, basically proved that the ether didn't exist, or proved to a very high degree of certainty that the ether did not exist. Essentially, the experiment had to do with measuring the relative speed of light on one side of the earth compared to another or when the rotation of the earth was going one way versus another. It's a bit complicated, but the basic idea was that you should have observed some

change in relative motion of the earth compared to the ether in different variations of the experiment and no change was observed, and the experiment was sensitive enough that it should have picked up that variation if it existed, and so the fact that no variation in speed or relative speed was observed is evidence that the ether, in fact, does not exist. So that was a massive blow when that was discovered in the late 19th century for the wave theory, because the wave theory worked on all these other fronts, you know, diffraction, interference, refraction, and so on, but there's no medium for it to travel, for the waves to travel through, so how does that work? How can you have a wave without a medium? So that was sort of left unresolved for a while,

and we'll come back to that point shortly. So that now, we've just covered the particle and wave theories of light, the sort of two classical theories. I now want to take a step a little bit backwards before we pick up again from the Michael and Moore experiment and talk a little bit about electromagnetism or electromagnetic theory. Now, electromagnetic theory was first sort of developed by Michael Faraday and later by James Maxwell in the mid-19th century. Basically, basically what Faraday discovered in 1845 was that a magnetic field could affect the plane of polarization, so like sort of the direction of polarization, of polarized light. It could rotate that plane of polarization. So this demonstrated some sort of interaction between magnetic fields and light, which was the first

time such an interaction had been observed before. And this was evidence that light was related to electromagnetism, which is not something that had previously been thought about too much. So this caused Therody to sort of get thinking, and a couple of years later he proposed that light actually was composed of high-frequency electromagnetic vibrations, which could propagate even without a medium like the ether. So this worked by Faraday inspired another scientist, very famous one called James Maxwell, to further study the relationship between electromagnetic radiation and light, and he sort of built the model that's really still used today, called Maxwell's equations essentially,

that describes how self-propagating electromagnetic waves could travel through space at a constant speed, and they could propagate themselves and travel even without a medium to travel in. And so from being able to develop this theory and also from Faraday's work, he, Maxwell concluded that light was a form of electromagnetic radiation. It wasn't just related to electromagnetism, it was electromagnetism in a sense. And these theories were essentially confirmed soon afterwards, by Heinrich Hertz, another famous physicist, who was able to generate radio waves in the laboratory,

detect them, describe their behavior, and they behaved exactly like Maxwell had described them, and they also displayed all the behaviors of visible light, like reflection, refraction, diffraction, diffraction, interference, and so on. So, you know, essentially, by around the 1880s or late 19th century, we had this theory of how light could, how electromagnetic radiation could behave exactly like light, how they could be self-propagating vibrations of electromagnetic fields,

which would then travel through space at the speed of light, and exhibit all the behaviors that we associate with light. So basically, just as the traditional particle and wave theories of light were failing, Newton's theory because it was inconsistent with the observation that light travels slower and denser mediums, and the Christian Huygens' wave theory because it predicted the existence of the luminiferous ether, which turned out not to exist as the Marksland-Morl experiment,

shown. So just as these two traditional models were failing, the late 19th century saw the electromagnetism arise as sort of a third theory of how light could work. And electromagnetism was more closely related to the wave theory of light, but it's a substantially different idea, in particular because it incorporates electricity and magnetism, which Huygens didn't have any conception of being related to light when he proposed his wave theory, and also because they did away with the need of the concept of the ether and just had light as a self-propagating wave. Now, I haven't described in very much detail what I mean by a self-propagating wave. That's because I want to say that for a later episode when I talk about electromagnetism in more detail. So I'm sort of not actually going to

answer the question of what light is completely in this episode. I will talk a little bit more about it, but what I wanted to focus on in this episode is just sort of the basic behavior of light. So I'm not going to go into detail about the actual electromagnetic properties of light. That's for a future episode. But I do need to mention this electromagnetic theory to sort of complete the intellectual history of light and to explain what I'm now going to move on to, which is wave particle duality. Okay, so by the early 20th century, basically, the corpuscular theory of light was dead. Every problem, it seemed, with the wave theory of light had been eyed out. It explained, you know, reflection, refraction, diffraction, interference of light.

We had a complete theory of electromagnetism, which was well established theoretically and experimentally, which explained how light could propagate through space at exactly the speed that it was predicted to, speed of light, and how it could, also we had a theory of how it could propagate without needing a medium, so it didn't matter that the ether didn't exist, and it seemed all as well and good, we didn't need the particle theory anymore, light was a wave, end of story. However, a certain individual named Einstein, sort of threw a spanner in the works, when he resurrected the particle theory of light, to explain the photoelectric effect. Now, I'm not sure if I've discussed this before, but the photoelectric effect essentially refers to the excitation of electrons by absorption of light.

So, reference back to some earlier episodes, for example, the history of the atom, or principles of quantum mechanics may be useful if you are a bit hazy on some of these concepts. But basically, what had been observed was that when light, or of a certain frequency, was shined on certain metals, the metals would begin essentially conducting electric current, or electrons were observed to be flowing throughout the metal, or given off by the metal, and it wasn't really understood why this was the case. That is the photoelectric effect, and Einstein was trying to explain why this occurred. Why is it that when you shine light on a metal you get electricity, you get electrons moving around, not what's with that? He proposed that what was happening was that the electrons themselves were absorbing the incident light, which came in as discrete particles, as per Newton's theory. So the light comes in as single packets called photons, which were then absorbed by the electrons,

and causing the electrons to gain energy and become excited and therefore move around the material. And this theory turned out to be very successful in explaining the photoelectric effect and was sort of demonstrated experimentally and so on. So once again, after around 1905 with the photoelectric effect explained by Einstein, we seem to be back in our old problem of, is light a particle or a wave? It seems to be sort of both. Electromagnetism says it's a wave, but Einstein, and the photoelectric effect says that it's a particle.

This was only finally resolved with the development of quantum mechanical theory, sort of around the 1930s, which reconciled the two natures of light by introducing the concept of particle wave duality, wave particle duality, which I've talked about in the quantum mechanics episode. So basically this means light is both a particle and a wave, and different experiments or different ways, different modes of interactions with matter will sort of yield one property or the other. So it sort of depends upon how you measure electromagnetic radiation basically, or it depends how you measure light as

to where you get wave-like or particle-like behavior. Light itself is made of photons, which are discrete tiny packets, sort of like Newton's corpuscles. So these are photons, these are particles. However, these particles also exhibit wave-like behavior, so they exhibit interference, reflection, diffraction, and so on. And so the fact that light exhibits both of these properties is called wave particle duality, and it's explained by quantum mechanical theory. Once again, see that episode. Okay, so that's sort of the history of light, the particle wave duality problems, and the electromagnetic theory and how it seemed to explain it, then the modern quantum mechanical wave particle duality theory, which has sort of put an end to that particular

conundrum. Light's still a very interesting phenomenon, though, so now we're going to move away from just looking at what light is to looking at how light interacts with matter and some of the behaviors of light. First we'll talk about media boundaries. Now, generally the interesting phenomena that we're concerned with about light is when you move from one media, which is just one substance that's transmitting the light into another. It passes between one medium to another. So that could be from water to air or from air to water or from glass to rock or glass to air or anything, just two different substances made of different materials, having different physical properties. When light moves from one of those to the other, that's called a changing media.

passing through the boundary between the media, that's where the interesting stuff happens. When you're just travelling through a media, generally a medium, generally light just goes into straight line at the speed of light or at least at the speed of light in that medium, because the speed of light actually changes depending on the medium you're travelling through. And it continues to a straight line and doesn't really do anything. It's only when you have generally boundaries between media that the interesting stuff happens. Well, there is going to be some absorption if you're travelling through certain media. But at a first approximation, most of the interesting stuff happens at boundaries between media.

Okay, now there's a concept in regards to this called refractive index. Refractive index essentially means, well, as you can maybe guess from the name, it's an index, so it refers to how much the material refracts light when light passes into it. Now remember refraction is just the bending or the changing direction of light when it changes from one medium to another, and some materials cause more refraction than others, and so we measure the relative amount of refraction and combine them together and call the result a refractive index. The refractive index, the high the refractive index of a material, essentially the slower light travels in that medium. And so the bigger, therefore, the change in speed, the more refraction there is. So that's why

dense materials tend to be associated with slow light, tend to be associated with high refractive index. So when light passes from a vacuum into air, air is slightly denser than a vacuum, but really not very much. So its refractive index is only slightly slightly high than that of a vacuum, which is defined to have a refractive index of 1.0.0. Sorry, air has a refractive index of like 1.001. It's very close to one, but a little bit higher. So that means light travels a little bit slower in air than in a vacuum, and it's refracted a little bit if it passes from vacuum straight into air, although I don't really know how that could happen.

But anyway, things like crystals or diamond, I think diamonds, for example, have a very high refractive index. I think it's like 2.5 or something. So that means light travels about two and a half times slower in diamond than it travels in air or vacuum. And when light travels between one from one medium to the other, there's a substantial degree of refraction or degree of bending because the diamond has a high refractive index. And the refractive index essentially just depends on the material that the substance

is made of. It may also depend upon temperature and some other things, but generally it's mostly, it's most of the material that the physical structure and the atomic bonds and all that sort of stuff will affect the refractive index and therefore the speed of light in that material. The higher the refractive index, the slower light travels through a substance. So people talk about the speed of light, which is about 300 million kilometres per second. It's slightly miscellaneous because light doesn't have a single speed. Light really can travel at any speed up to the speed of light, so-called. The speed of light that we talk about refers to the speed of light in a vacuum. That's, you know, when you have a refractive index of one. So the speed of light in vacuum is basically the same as the speed of light in air,

but the speed of light in quartz or in diamond or in water is going to be slower than it is in air or vacuum because those are denser materials. The denser the material, the slower light will travel. So light doesn't always travel at the speed of light. It never travels faster than the speed of light, at least as far as we know, but it can travel slower than that if it's travelling through a denser medium. That's important to keep in mind. Okay, so when this light, or when the photons, sort of they're traveling through the medium and then they hit the boundary between this medium and the next one, so when they actually get to that boundary, there's essentially three main things that they can do. The light can either be reflected,

it can be absorbed, or it can be transmitted. And I'll talk about each of those in turn. Well, not in turn. I'll start with absorption. Absorption essentially means that the lights swallowed up in a sense. The photons are going through and they hit something that absorbs them and then the photons are gone. They may be remitted at a lower wave, excuse me, at a lower frequency, then they're sort of a different photon because they've got different frequency and therefore a different level of energy. But at least part of the energy of the photons is eaten up in a sense when you're absorbed. So you can think of the material swallowing up the photons or at least swallowing up part of their energy. That's what absorption is. A pigment

is any material that absorbs at least some of the light that's incident. upon it. By the way, I should introduce terms here, incident light just means light that's shining on a given surface. So, you know, if you hold up your hands to the sun and you feel the warmth of the sun, those sun rays are light that is incident upon your hand. It's just shining on it. So some of those incident rays will be absorbed by your hand. And the materials that do that, that absorb the light are called pigments. Pigments occur in the skin. That's, you know, we say the skin has a certain pigment. It's just referring to essentially the proteins that are absorbing certain frequencies of light, causing your skin to appear a certain colour.

Pigments are also used in paints. So the particular pigments that you put in the paint affect the frequencies of light that are absorbed by the paint and therefore the colour of the paint. Different materials will absorb different wavelengths of light or different frequencies of the incident light. Remember, wavelength and frequency are related to each other. And frequencies also related to the energy of a photon.

So the colour of something is essentially going to be determined by what pigments it has in it, what materials it has in it, and what frequencies or wavelengths of light they absorb. If a substance absorbs all wavelengths of light, or at least all wavelengths of visible light that are incident upon it, then no light is reflected, or very little light is reflected to us, and essentially the object appears black to us. Because humans can only see light that's reflected off an object. So the only reason you see anything is because light shines from some source, bounces off the object and reflects into your eyes and you see it. If all the light is absorbed,

or even if all the light have transmitted, but we'll come back to that, If all the light is absorbed by a particular object, none of it's reflected, or even if not enough of the light is reflected, you can't see it, and therefore it appears black to you, or just invisible. Not invisible, it's going to appear black if you won't be invisible, because you won't be able to see through it. So that's what black objects are. They absorb all incident light upon them, at least in the visible portion of the spectrum. Now, why is it that some materials absorb some wavelengths of light, and some materials absorb other wavelengths of light, and some materials don't absorb any wavelengths of light? So glass, for example, doesn't really absorb much of any wavelengths of light. That's why we can see through it.

Why that difference? Well, essentially, the difference is because, remember, I said that when a material absorbs the photons or absorbs the light, it sort of swallows them up. There is a principle called the conservation of energy. So if the material is swallowing up photons, it's swallowing up energy. So that energy has to go somewhere. It can't just disappear. Where does the energy go?

Well, essentially, it goes into the vibrational motions of the molecules or the atoms inside the molecules. Or maybe it goes into the motions. of the electrons in the electron clouds or the rotational motion of the atoms or something like that so there's a variety of exact ways you can go in but basically it's sort of the

internal vibrational and rotational energies of the molecules and atoms in the substance that's where the energy from the light goes but only some combinations of vibrations or frequencies of vibrations and rotations and so on are going to be stable other ones just won't work they would cause the molecule

to shatter apart or would cancel themselves out if you remember from the episode about vibrations and waves, this is essentially an application of resonance. Basically, the incoming energy from the swallowed photons has to be in resonance with the vibrational and rotational motions of the atoms and the molecules of the substance that's doing the absorbing. If the incident energy is not in resonance, then it won't be able to be absorbed by the system, and so the photons won't be absorbed, and absorption won't occur. But to be in resonance with the internal vibrational motions of the system,

you have to have the right wavelength or the right frequency. If you don't have have the right frequency, then you won't be in resonance and you won't be absorbed. So essentially, you can think about absorption occurs when there's a match between the internal, vibrational, rotational, and thermal motions of a system of the material. There's a match between that and the frequency slash wavelength of the incident photons. When there's a match, they can be absorbed. When there is not a match, the photons will not be absorbed, and absorption does not occur. Now, don't get confused. This is not the same as the photoelectric effect. The photoelectric effect occurs when the electrons themselves absorb the energy of the photons and are promoted

to higher energy states inside the shells of the atom or even completely kicked off from the atom and freed from the atom. That's different. That's the photoelectric effect. So that's the, with electrons. Absorption can relate to electrons, but it's not just the electrons absorbing the energy. It's also, it's the atoms and molecules themselves, so it's a broader process. So don't get confused. Those processes are sort of similar, but a bit different. So you can have the photoelectric effect occurring with no absorption, vice versa, because they're independent. Okay, so that's absorption. When you have a match between the energies of the incident photons and the energies of the material that's doing that the light is incident upon, when there's a match, you can get absorption,

and the internal vibrational energies or rotational energies of the material increase, and the photon is either completely absorbed, or it might be re-emitted at a lower wavelength, because you've removed some of the energy from it. The next, so that's one thing that can happen to light absorption. The next thing that can happen is reflection. Remember, I said that reflection is what enables us to see objects. You can only see something if light is reflected off it and into your eyes. Reflection is sort of simple, basically. You know, the light rays or the photons travel towards the substance, they hit it, they're reflected off and they travel backwards. So in that sense, reflection is fairly simple. The light comes in, bounces off,

and travels back in the opposite direction. And you can think of that, reflection happens in both particles and waves. So whether you think of light as a particle or a wave, reflection sort of works. However, if you think about reflection at a sort of microscopic or an atomic level, it's a little bit harder to visualize because what's happening is you've got a photon or a wave particle photon thingy coming in and it's interacting with the atoms in the material somehow and that's causing that interaction is then causing the incident photons to travel back in the opposite direction that they came how that happens is actually very complicated as i found out when i was researching for this it's studied in a field called electrodynamics which is a fairly uh sort of esoteric field of physics

not exactly esoteric but it's difficult um related to quantum mechanics uh so maybe i'll do an episode on that in the future, but basically the idea is that there is an interaction between the incident photons and the outer electrons of whatever material that the light is incident upon, that interaction can cause the incident photons to be essentially ejected in the opposite direction they came. So we call that reflection when that occurs. But it doesn't always lead to that. Remember, we saw from just when I talked about absorption that sometimes the incident photons can be absorbed.

As I'll talk about later, the incident photons can be transmitted. So whether or not the incident photons are reflected depends. upon the precise electromagnetic properties of the material that it's hitting. So this is why metals, which are good conductors of electricity and therefore are also good reflectors of light essentially, tend to be shiny because they're reflecting a lot of light, whereas ionic crystals or non-metallic elements that tend to be insulators, they don't transmit electricity very well, they tend to be transparent because they cannot reflect very much light. Their outer electron structures are not very good at essentially maintaining the interaction between the electrons in the outer shells and the photons

that are coming in, in ionic crystals, you can't get that interaction very well, which reflects the electrons, and so you don't get much reflection in those sort of things. So that's why, you know, crystals or diamonds and stuff like that tend to be transparent or translucent, because you're not getting very much reflection off them. You certainly get, you generally always get some, but not nearly as much as you would from, say, a lump of coal or a metal or something. So there's a couple of things going on in reflection, because when you think of reflection, you might think of a mirror when you see a nice clear image of yourself. That's one type of reflection, but there's also just whenever you look at any object that you can see,

I'm looking at my keyboard right now and it's black. I can see that because of the reflection of light shining off the keyboard. So that reflection is sort of the same thing as the reflection when I see myself in a mirror. But I don't see myself in the keyboard, but I see myself in a mirror. So what's going on there? So there's two things going on simultaneously. One is, will the material reflect light at all, or to what extent will it reflect light? metals tend to be good reflectors, crystals, not so good reflectors.

The second question is, once we found out whether the material reflects light, the next question is how will it reflect it? Will the reflection be specular, which is like a mirror, producing a nice, crisp image, or will it be diffuse reflection? Which is still reflection, but it loses the coherence of the light rays. And so they go off in all directions and it scatters the light rays so that you don't have that clear image. So two types of reflection,

specular and diffuse. A mirror produces specular reflection. It maintains the relative angles and positions of all the light rays or all the photons when they're reflected. And so sort of the, you think about it, what happens is the photons hit your face, and then they're all reflected off with certain angles relative to each other, and positions relative to each other, and then they travel and hit the mirror, and then the mirror has experienced a specular reflection, so it maintains all those angles and relative positions in someone of the photons, which then bounce back and hit your face or go into your eyes. And so you see, see a picture of yourself. When you have diffuse reflection, essentially the same thing happens. So,

you know, I've got the light, it strikes my face, reflects off. I've got a certain angles and positions of the photons of my face, and then the light from my face hits the wall, but the wall does not experience a specular reflection. And so essentially, it does reflect the light from my face, but it changes all of the angles and relative positions of the photons, and so it sort of scrambles them up. And so when they travel back to my eye, all of that information that is, you know, is embodied in the relative angles and so on of the light rays and of the photons is all mixed up and jumbled up. And so in a sense I'm seeing my face, but it's a complete mishmash of all the different parts.

And so I just see the wall. I don't see my face anymore because then information's been lost. That's the difference between specular and diffuse reflection. And whether a material experience of specular or diffuse reflection depends largely on its microscopic structure. If surfaces are very smooth, they'll tend to maintain the relative angles of the incident photons and therefore just sort of reflect them off as a group, maintaining their relative positions and angles, and therefore maintaining the image, maintaining the information in that

light. However, if the surface is rough and sort of is jagged and some bits point this way and some bits point the other way, then the exact orientations of the light that's being, when it's bounced off, will change and therefore you lose that nice image. It's a little bit easy to see this if you see a diagram. If you just, like Google, diffuse versus specular reflection, you should see fairly clearly what I'm talking about when I say. that the angles are changed by a broken-up irregular surface. But remember, specular and diffuse reflection are still both reflections. So in both cases, you've got that complex interaction of the incident light with the outer shell of electrons, electromagnetic process, causing the

light to be reflected. It just depends upon the microstructure of the material as to whether the angles between the photons are going to be preserved or not, and therefore whether the reflection will be specular or diffuse. Okay, so that's reflection when the light bounces back. We've talked about absorption when the light's swallowed up, there's one other possibility, which is transmission. Transmittance, or transmission basically, it refers to how much light travels through the medium. It just passes completely through it without really interacting at all. So transmission is sort of like the leftover category. If light is not either absorbed or reflected, then sort of by default, it's transmitted. You may have heard of neutrinos, which I haven't

talked about before on the podcast, but neutrinos are very small, very light particles, which interact sometimes with matter, but most of the time they don't interact. So what I mean by that is the sun is giving off billions of neutrinos every second, which are passing, most of them just pass straight through the Earth. Why? Because neutrinos don't really interact with the Earth. A few of them will, and they try and detect them in using big vats of cleaning chemicals and whatever under the Earth, but they're very hard to detect, because they don't interact very much.

Mostly they just pass straight through. Why do I mention this? Because sort of passing straight through was like the default situation of matter. Remember, it's one of Newton's laws. I think it's the first law, essentially inertia. A particle will just keep going in the same direction at the same speed unless some force acts on it. So, you know, if the light's just passing through,

and for whatever reason, none of the electrons or atoms that it's passing, none of them absorb it, none of them reflect it, it just keeps going and transmits right through. So that's why I say transmission is sort of like the default or the third category where neither of the other two things happen. A material will transmit light will pass light right through it if it can neither absorb nor reflect the light. in most cases it will absorb and reflect a little bit of the light, but not very much,

and so most of it passes straight through. Now, obviously, a classic example of a transparent material is glass. That's a very good transmitter, because it doesn't really have any pigment, so it can't absorb the materials. It can't absorb the light. That means the materials or the structure of the atoms inside it are not such that you can get that resonance effect between the energy of the incoming photons and the internal vibrational, rotational structure of the atoms and molecules.

So that doesn't work. It also, the outer shells of the electrons are arranged such that you can't get that interaction effect between the incident photons and those electrons, and so you can't get that reflection behavior. Neither of those are possible, and so the light just travels straight through. Now, there is generally a little bit of reflection, especially on the very surface of glass, and you have to be careful because if glass is not, glass is a crystal structure, but you can have different, I guess, types of glass, where the whole thing is one large crystal structure, which is sort of smooth and homogeneous, in which case the light can sail through. or it can be, it's still glass, but it's divided up into many tiny sort of microscopic crystals which then have internal boundaries relative to another. If you have that second case with lots of small, tiny boundaries between crystals within the light, sorry, within the glass, then light will essentially be bouncing around.

Every time it passes from one crystal to another, it essentially hits a new medium, and so some of it's going to be reflected, maybe a little bit's absorbed, but the thing is if that happens, the light's not going to pass nice and cleanly through the glass. It's going to, some of it will come through, but it's essentially going to look something like, something like those, I don't know exactly what they're called, but that glass, the light comes through, but the detail of the picture is lost, the sort of the translucent type glass. One reason of that, one reason for that could be that there is a lot of tiny internal crystals within the glass structure that's breaking up the clear pattern of the light.

So a traditional glass window you can just see right through depends not only upon lack of absorption and reflection, but it also depends upon there being not lots of little microchristial structures within the glass which would disrupt the light if it had to pass through them all essentially. Now, another thing to understand is how mirrors work. Now, mirrors work because they are made of shiny metal. Remember, metal is a good reflector, and if it's nice and smooth, it also is a specular reflector, so you see an image of the light just as it was incident upon the metal itself. mirrors are usually covered with glass, but the glass itself is not what does the reflection.

The reflection, the production of the image is solely the, as a result of the metal, the shiny metal behind the glass. The glass really just protects that. So when the light is hitting the mirror, it travels straight through, it's transmitted right through the glass, is reflected, specularly, off the metal at the back, and then comes back and hits your eye. So glass is not, at least the kind of glass you have in a mirror, is not actually what does the reflection.

It's the metal behind that. You can't have funky stuff happening when you have like two-way mirrors and so on, which maybe I'll talk about later, but I'm just sort of talking about the basic situation here, normal glass and normal mirrors. So I've now covered the three different types of behaviour essentially when light passes through a boundary between two media. Absorption, reflection and transmission. There are different classes of objects which essentially undergo these behaviors to different degrees.

A transparent object transmits all incident light with maybe a little bit of reflection. So that's why when you, if you look through a window, you can often just see a little reflection of yourself, depending on how the light's situation. That's because some of the lights being reflected, but most of it's being transmitted. A translucent object transmits some of the light but absorbs most of it. So that's like a frosted glass. That's the word I was looking for, frosted glass. So a translucent object is one you can sort of see through, but not very well. An opaque object is one that absorbs or reflects all incident light, but has little or no transmission. So a white opaque object is one that reflects all incident light, a black opaque object is one that absorbs all incident light,

and a coloured object will absorb some light and reflect some light and generally not transmit very much. Now, invisible objects would have to either not interact with light at all or cause light to bend around them. So I suppose in a sense a window is an invisible object, that would be a case of light not interacting with it at all. It just sails straight through, although even there's going to be some degree of interaction

because of the little bit of reflection you get and so on. Another possibility for invisible object is if the light bent around them and then return to its original orientation, so you didn't see the object at all. That's mostly theoretical, although they have done some laboratory work on actually getting that to happen, although mostly I think in radio waves, or maybe it was micro waves, not in the visible light spectrum. I haven't actually talked about what that is yet, so if you don't know what that is, don't worry. So they are sort of working on that, getting light to bend around things. That also happens in general relativity as a result of mass bending space time,

but that's another thing I haven't discussed yet, so that's for a future episode. But bear in mind, if an object was to be invisible, either it doesn't interact with light or light bends around it. Transmitting or reflecting or anything like that is not going to make an object invisible. Okay, so having covered that, I now want to talk a little bit about color, which I've touched on before, but I want to go into a little bit more detail now. Now, the color of an object is, so the color literally just refers to our sort of perception of what an object looks like.

It's actually very hard to define what that means exactly, but the color of an object is determined by the wavelength of light that it reflects. this is a little bit confusing. So for example, you would say that a plant, most plants are green, quote unquote, are green. However, the reason that they appear green is because they reflect green light, which means they absorb blue and red light, which are different wavelengths of light, because each colour has its own associated wavelength with it. So red light has a, red light light just literally is light of a particular wavelength. Blue light is light of a different wavelength, green light is light of a different wavelength again, and same with yellow and so on. So green plants are green,

because they absorb blue and red light, and they reflect green light, and so that's how they appear. So essentially, when we say plants are green, that almost means the exact opposite. It almost means that plants are the opposite of green, because that's the things they absorb. They absorb the blue and the red light, and they sort of throw away the green. The green's what's left over, and so that's the color they seem. So whenever you see an object, that color that you perceive the object as is the color of the wavelength of light that is being reflected by that object. anything that's absorbed by that object, you won't see, and therefore the object doesn't appear that colour. So that also means that when you illuminate an object with different colours of light,

it can change its appearance, because it can obviously only reflect the light that's incident upon it. So as an example, if the object appeared blue under normal light, it would appear black when viewed under yellow light, because it doesn't reflect yellow light, and therefore if that's the only thing that's incident upon it, it appears black because it doesn't reflect anything. Now, speaking of white and black, I said that every... color has its sort of own wavelength of light associated with it, or frequency of light associated with it. White and black are exceptions, though. White is merely the interpretation that

our brain renders to the situation where we see all the colors at once reflected or emitted by an object. So, whenever anything is white, that means it's reflecting all, or at least most of the wavelengths of light that we can perceive. When something is black, as I've mentioned before, that means it's reflecting none of the wavelengths, or virtually none of them, and so we can't see anything from it. And also, you remember I said that different, objects will look different under different types of lighting. That is why objects can look very different under fluorescent lights compared to just normal light bulbs, compared to sunlight, and maybe compared to sunlight at noon, compared to early morning and evening and so on. It's because if the incident wavelengths have

different colours, then the objects themselves are going to reflect different colours, and so the objects can look different. So if you've ever thought that before, it's actually not your imagination, it's legit. Or it could be imagination too, I suppose, but there's physics behind it too. Okay, so that's colour. Color refers to wavelengths, essentially. One last thing that I'd like to talk about is refraction, which I've sort of mentioned a bazillion times before already in this episode and the previous one. Remember, refraction is the bending of light that occurs at the interface between two transparent media. It occurs because the speed at which light is travelling changes it between the two media, and so the direction of the waves changes, and so the direction of the light

rate changes. As I've also mentioned, the index of refraction determines how much the bending, how much light is bent, how much the speed is changed, and therefore how much it's bent. The direction that light is bent in depends upon whether the light is slowing down, that is travelling from an index with a lower index of refraction to a material with the one with a higher index of refraction, that will mean the light's slowing down, or vice versa, when the light is travelling from a medium with a high index of refraction to one with a lower index of refraction, case the light is speeding up. So if the light's going to a, if the light's going from low to high index of refraction, that means the light is refracted towards the normal, where the normal is a line

that's perpendicular to the surface of an object. So you can think of the normal as like a line, an arbitrary line just sticking out of the surface that light is hitting. That's the normal. So when light travels from low to high refractive index, the light's refracted away from the normal. When light travels from low to high refractive index, the reverse occurs and the light is refractive. away from the normal. So don't worry too much about keeping that in your head. It's a little hard, especially when you can't actually visualize it, but I just wanted to put that idea out there, that the direction of refraction depends upon whether you're going from low to high speed or high to low speed. Dispersion is another interesting phenomenon that I wanted to talk about, which is closely related to refraction.

Dispersion is the name giving to the splitting up of white light into its component colors or component wavelengths when it's passed through a prism. So remember, white, or white light is just light that's composed of all of the wavelengths that we can see. So that means if there's a ray of white light, it's actually got all those different wavelengths inside it. And when you pass white light through a prism, it turns out that you can spit up those wavelengths so that we can see them separately from each other. And that process is called dispersion. I believe, I don't know if Newton discovered this, but he certainly described it and talked about it. The reason for this is because different colours have slightly different refractive indexes.

That is, different wavelengths of light travel at slightly different speeds in different materials. In fact, red tends to refract the least while violet refracts the most. And the reason for this in turn is because different colours are associated with different wavelengths and the wavelength will determine how much refraction you experience, how much of a change in direction there is, and therefore if different colours are being refracted or bent different amounts, when you pass the light through the prism, the bending is different for each wavelength, and so the lights are sort of split apart from each other and you can see them independently. And this phenomenon of dispersion is responsible for the rainbow,

essentially because the raindrops act like lots of little tiny prisms, which are causing the refraction of sunlight and therefore that's split light into its component colors and you see a rainbow. I'll probably talk about it in a little more detail exactly how that works, but that's the basic idea. So that's it for this episode. In the meantime, while you're waiting for the next episode, you can go onto iTunes and post a review of my podcast and a rating. I've got a few so far, but more ratings are always helpful. You can also spread the word about my podcast by telling all your friends how interesting it is, or family members or random strangers even if you like. Send me an email at Fods12 at gmail.com.

If you want to get in touch with me about the podcast, ask any questions, give feedback. Suggestions for future topics are also welcome. Just the other day I received an email from one Tim Koch. Hope I'm pronouncing that name correctly. Who gave me some suggestions for future topics, which I will look into. That's actually the first email I've ever received suggesting future topics. So if you have a particular subject you want to learn more about, send me an email, and it's quite likely that that will actually have an effect.

on what I decided to do next. So that's it for now, and I'll talk to you next time. Just a brief postscript, I think I said it a couple of points in this episode that the speed of light in a vacuum was equal to 300 million kilometres per second. The correct value is 300 million meters per second, not 300 million kilometers per second. So the value in kilometers per second is 300,000 kilometers per second, which you can remember by the fact that light takes approximately one second, one and a bit seconds to go from Earth to the Moon, and that's about a dissoning. of 300,000 kilometres. Apologies for that error.