Why is light slower in glass? - Sixty Symbols

okay so here's the situation two sixty symbols professors professor Moreau field and Professor Moriarty both got in touch saying they wanted to talk about why the speed of light seemed to slow down through glass essentially about refraction so normally what would happen I'd interview both of them do some editing chop it all together and make one video but we're going to do something different this time I'm going to show you both interviews pretty much unedited except for a few little cosmetic changes and cutting out a few of my questions where you didn't really need to hear my question I'm going to let you watch them both this video is Professor Moreau field and very shortly you'll be able to watch the one from Professor Moriarty I want to do a video about refractive index in because it actually has a symbol apart from anything else but also because it's something where we've done quite a lot of videos which mention refractive index and we talk about the speed of light when it's going through a medium through glass or something like that and we talk about the speed of light slowing down and invariably when we do that there's this sort of firestorm in the comments to the video all about people say ah but photons always travel at the speed of light what's going on why does the speed of light to slow down and so on so I wanted to try and explain what's going on and the fact that the speed of light really does slow down when you're going to a medium in crude terms if you measure the speed of light in a vacuum you always end up with the same answer three hundred thousand kilometers per second well they're about C okay if you measure the speed of light in something else air glass whatever it is you'll get an answer the refractive index is just defined as the ratio of those two numbers so for example the refractive index of glass is typically around 1.4 which means that light travels about 40 percent slower in glass than it does in the vacuum that much yeah I always imagined it was just like a miniscule chair no so in air it's like it's you know it's a fraction of a percent so air really is the speed of light really is very close to the speed of light but in us in a fairly dense material like glass yes it's about 40 percent slower in the glass than it would be in a vacuum so I mean it's responsible for all sorts of things the reason why when you look through glass you know when you look at straw in a glass of water you see that the straw appears to be bent and so on so all these things to do with the pars of light being bent are all to do with what happens when light goes from one refractive a medium with one refractive index for a medium with another refractive index so from for example from air to glass or a hair to water wear glasses I do so and you know they wouldn't work if it weren't for refractive index because that's what bends the light right and so actually yes lint the lenses wouldn't work so lots and lots of physics really depends on refractive indices so this always comes out with people's a table well you know so people are quite reasonably happy with the idea of waves and maybe the waves slow down but then you start thinking about photons particles of light and there's this view that a photon always travels at the speed of light and so when people start thinking about this and say well how can it be when you've got your photons going through if you think of the photon picture when you got your photons going through glass why that they take longer to come out the other side so the picture a lot of people have let me draw a picture for you so the picture a lot of people have is I've got a material rather than a vacuum so I've got a lot of atoms in there we atoms and make up and maybe they'll be in a lattice if it's a solid or they'll be randomly distributed if its water or but there'll be just a bunch of atoms so the picture a lot of people have is my part of God light comes in and kind of does a sort of pinball pinball that's the what I was in ok so so a picture a lot of people have is that the light comes in and does it sort of pinball thing of bouncing around okay and then coming out the other side and that sort of makes sense right because the if this thing is still traveling at the speed of light obviously now it's travelled a longer path to get out the other side and therefore it should take longer to come out the other side the problem with that is if you think about it if take these glasses make glass as an example where you've got this refractive index of around 1.4 that means that the path or thing has to travel along has to be something like instead of travelling straight through it has to travel you know forty percent further that means it has to be sort of traveling on pause like this right in order to come out the other side for forty percent slower because that's the angle that typically you have to be going for this path to be about forty percent longer than the straight through path okay okay the problem with that is that that would mean that some of the light will do there and some of the light will come in and bounce this way and this way and this way and this way and then end up coming out that way so what you'd expect for the light coming out the far side in this case is that the light should all be heading out in all sorts of different directions just depending on what it has to bounce off lost and of course there's not what happens if you shine a laser into a piece of glass then the laser beam comes in one side and it comes out the other side and it's still a laser beam still all pointed in the same direction so this picture really doesn't work of thinking about this game of pinball going on because if you start doing that actually mean that the light really should be spreading out in all directions but couldn't I think of it more as instead of pinball wading through treacle or honey and I'm just walking more slowly because it's harder for my legs to move through this viscous material but then you have to okay so that's fine but then you have to get okay so that's sort of a macroscopic you know let's go down to the microscopic view what's happening what's the light doing when it interacts with the atoms and there is another possibility right the other possibility that could be going on here's again here's a bunch of atoms and it could be that the lights coming in it's getting absorbed by one of these atoms it then sticks around for a little while until all the atoms uses to reom it the light again and send it on its way and that's sort of at the microscopic level that's this sort of what's going on in a treaty picture that things are just being delayed as they're going through again there's a couple of problems with that I mean we know that atoms do absorb atoms on their own absorb light so this picture sort of works that atoms do absorb light and then they'll remit it sometime later there's two problems with that firstly atoms tend to absorb light at very specific frequencies and so you would then expect your refractive index to affect those particular frequencies a lot because the atoms are absorbing light those particular frequencies and other frequencies not very much at all so that means that the refractive index should vary dramatically depending on exactly what wavelength of light you're looking at it doesn't happen that way refractive index does depend on the wavelength of light but generally rather smoothly rather than this very definitive screen way the second problem with that is that that is again fundamentally a kind of a stochastic process of random process how many atoms did you happen to bump into in your way through how long did each atom happen to delay the light for because how long a an atom takes to when it absorbs a photon of light how long it takes to remit it again it's just a random process that means that how long it takes a photon to get through should just depend in a fairly sort of random way sometimes they'll get through very quickly sometimes they'll have more a problem again that's not what you find when you measure that how you know you first turn on a light through a block of glass and measure when the first light comes out the other side it's always the same amount of time so again you can't have any kind of stochastic process like that going on all right then you've convinced me that they're wrong tell me what's right all right this is where life starts getting very complicated but yeah that wasn't complicated enough this is where things do start becoming a little more complicated because now we have to start thinking always with it with these microscopic systems you can think about them classically or you can think about them in terms of quantum mechanics and at some level they're sort of both getting part of the picture it's just like you know you can treat like sometimes like a way sometimes like a particle so we need to think about both possibility so let's start with the classical end of things the wave end of things what you've actually got in your solid is a whole bunch of atoms arranged in the lattice as the light comes in as an electromagnetic wave okay it's basically a varying electric field that's going to jiggle all the atoms in this thing because they've got charges and so on so all the electrons are going to get pulled around by that electric field as it goes past and that's gonna when you then start moving these electric charges around that means that each atom is going to respond by producing electromagnetic waves of its own okay so the waves come in they start jiggling the atoms around which means that each of the ways will then start kind of oscillating in sympathy with the wave that's causing it and it's the superposition of the wave coming in coming through the undisturbed wave and all these sort of waves that are being created by the atoms that make up the material when you add all those waves together you end up with the final the wave that's actually propagating through the glass and that wave is the one that's then traveling at less than the speed of light okay so even if each individual wave that you're producing could be traveling the speed of light the kind of the superposition of them then create something that's actually sort of is 100 it hasn't traveled quite as far as you expected it to so the net effect of adding the light that comes in with all light that you end up generating through jiggling all the individual atoms produces this picture of a the overall wave that you end up with by adding all that lot together is traveling slower and the problem is if you actually want to do what you know I'm rather than just sitting here waving your hands about if I actually wanted to show you that actually the speed that the net result of adding all these waves together is to produce a wave which is traveling at less than the speed of light that's a lot of maths and so you can do it by solving the equations of what happens if I had all these different waves together each with different phase shifts and so on but it's just a mathematical mess to get to that point so when when Barry the beam of light entered a piece of glass and all and started making all the bits of glass start jiggling how did that then affect Barry the beam of light it didn't accept well some of berries let energy was lost because actually it was being used to jiggle all these other things around so some of that light would have been lost for that reason but it was then all these friends that Barry had created in the process and the sum of all those different waves then created the net effect the net wave that was traveling through the thing which was that they no wave which was traveling at less than the speed of light what is it about all those friends all that extra wave mnestheus some don to bury that him rather than enhancing him and making him faster in fact you can in very peculiar circumstances create things where you end up with a wave which is traveling faster than the speed of light in vacuum physicists tend to try and keep quiet about this because it makes life complicated and it turns out actually you don't end up you know everyone gets worried because at that point you end up saying am i breaking laws of physics and my braking relativity and so on actually you don't because then you start saying okay so can I actually travel to transfer any information through this medium can I use this to actually get information to travel faster than speed like the answer is you can't because there's two types of speed for a wave there's a things called its phase speed which is what I say I've been talking up to up till now about there's also a thing called its group speed and the group speed is if for example you wanted to produce a little pulse of light how fast would that pulse propagate through and the group speed and that's what you need if you actually wanted to send information because if I want to do Morse code or whatever I've got to send little pulses of light through it the group speed always ends up less than speed of light the phase speed under peculiar circumstances can end up faster than speed of light rather than slow and then there's be the light so that's the classical explanation okay the simplest version of the quantum picture is to say actually it's kind of the same thing because the weird thing so photons have really weird properties right for example if you do this experiment of having two slits and you shine light through it you end up creating an interference pattern just because the light that went through one fit slit interferes with the light that went through another slit so it's kind of the same mastery story we're talking about before that you actually end up with like carrying traveling through different parts interfering and producing a new phenomenon but actually if you think about things in terms of photons even if you have a light source which is so weak that only one photon is going through at a time you still create an interference pattern and the only way we can understand there is if the photon actually went through both slits so one particle manages to go through both slits at once and that's is this is getting us into this weird quantum mechanical world okay we can do the same thing with our solid of saying okay so if I think of my light as a bunch of photons I can actually think of a photon following every possible path through this thing and as it goes through it interacts with each of the individual atoms get sri radiated and what i see in the kind of the final light that this is producing is the the superposition of all those things the superposition of the light the photon that came in undisturbed but also all these other paths that it followed interacting with all the atoms on the way through and that really is basically the same as the classical picture where we've kind of each atom is sort of producing its own version of the light here it's just thinking about the photon following every possible path and then those photons that photon by following every possible path ends up interfering with itself and creating a net effect of light that's traveling slower than the speed of light where do you draw the line though if this if our photon is going through every possible way and we add up the superposition every possible way seems like an infinite thing it could have it is yes it's very clever how our photon manages to follow every possible path to get to where it's going nonetheless that seems to be when you do the mass of say what would I expect to see if that's the way that photons were behaving and then look at the real universe you find actually that's the way the real universe works I understand you gave to two scenarios that is classical one helped you know the jiggling and the adding of the waves and that and this quantum one which is a bit more mind-bending but nevertheless the maths works the maths works for both of them except that which one is actually happy but so that's the problem with physics right is that physics what we're doing is we're modeling reality there's one reality you can have multiple models and sometimes the models are exactly all you know as far as we can tell there's no you know the models are entirely right in other cases each mole kind of captures some aspect of reality in this case you can sort of explain the whole thing in terms of the quantum model there are things you can't explain in the classical model so in that sense the quantum model is more but actually the classical model is very helpful because it's much easier to picture and actually you don't end up with these mind-bending things of one photon following an infinite number of possible paths and those kinds of things so actually a lot of the time it makes sense to think about things in terms of classical physics just because it's soluble we can understand what's going on we have a physical intuition for what's going on but once in a while you have to delve into the quantum mechanical explanation for things just the messed things up still further there's another quantum way of looking at this which is this and again it's this question of coming up with ways of modeling things so I'll just talk about two ways of modeling things there's another way of modeling things that says actually a system that's a kind of a lattice or whatever that's in your your material and the light traveling through it is completely different from just light in a vacuum and so actually we shouldn't just think of you know light in a vacuum we can explain as a photon this combination of light plus the lattice and all the way that the lattice can be made to jiggle is a completely different system and when we come to solve that completely different system maybe we'll come up with a completely different particle rather than a photon as the as the particle that's associated with light and so physicists who study these systems have actually come up with a different way of mathematically formulating these things where there is a different kind of particle or thing called a polariton and a pallet on is it's this complex combination of the oscillations of the electromagnetic waves of the light and the oscillations of all the stuff in the material it's passing through combined together produce this kind of particle this kind of particle the play written has mass and because it has mass that means it travels it's less than the speed of light so if you want a different way of thinking these things but also quantum mechanical one is that your photon comes along when it enters this medium the maths that you're solving changes and so actually the particle changes and instead of being a photon it becomes one of these things a polariton the peloton bimbel's along a bit less than the speed of light and of course when it comes out the other side then once again you're solving the regular equations of light in a vacuum or light in air and that means that you can then treat it as a photon again and it scoots off at the speed of light again I told you the physics sleep yeah everything's your fault so if you'd like to see the Professor Moriarty interview that will be available soon unless you're watching in the future in which case it's available now links in all the usual places

Channel: Sixty Symbols

Views: 746,550

Rating: 4.9265924 out of 5

Keywords: sixtysymbols, light, refraction, photons, electrons, glass, speed of light, Slow Light

Id: CiHN0ZWE5bk

Length: 16min 30sec (990 seconds)

Published: Thu Jul 11 2013