we see incredible optical illusions all around us almost every day right but what causes them one of the main reasons is that when light goes from one medium to another like say from vacuum or air into glass it changes its speed because of which it bends this bending is what we call refraction so refraction is the phenomena where light bends when it changes medium and it happens because it changes its speed when it goes from one medium to another now one part of physics would be figuring out exactly how much it bends and then using that to figure out the geometry of like you know how we get the images why are the images big they're small and inverted and all of those cool things we call that the geometrical Optics we're not going to do that in this video because in this video we're going to try and understand why this happens a couple of questions that we could be having in our mind is that question number one we could ask hey why is it that if light changes its speed it should bend I mean for example when light enters glass it slows down but why should it Bend why can't it just continue in the same direction just a little slower why should it Bend and then we could have an even more fundamental question why does light change speed in the first place why does light slow down for example when it enters glass what's going on so let's try and answer these questions one by one so let's start with the first one why does light changing speed implies bending to answer this question let's remember our model of light that is it's an electromagnetic wave with oscillating electric Fields you can think of these as electric field vectors and also magnetic fields which I've not shown over here okay but let me also get rid of these electric field vectors and let me just look at the wave this is an electromagnetic wave now the key thing over here is that this light is a wave in three dimensions so if you were to sort of like look from the top we may see something like this this was the wave that we looked at earlier but now you can imagine a lot of these waves all stacked up I'll show some more of these waves but you can imagine like there are a lot of these waves all stacked up that's what you know this is how you can visualize this wave in 3D and you now get to see something that you couldn't earlier you can see these lines these lines represent set of all crests another set of all crests you also see here set of all troughs right we give a name to these lines we call them sorry we call them wavefronts okay what are wavefronts you can say wavefronts are set of all points that are in Phase with each other remember the word phase in Phase basically means that the electric field vectors over here are all oscillating in sync with each other they all finished the same number of oscillations at any given time think of it that way okay so set of all such points in phase we call them as wavefronts and over here we see these wavefronts to be parallel and straight and stuff but that may not be true WS can have different shapes but what's important for us is that the direction in which the wave travels will always be perpendicular to the wavefront you can see that over here right let me give another example let's take a familiar example of water waves if you're looking at water waves from the top you can now see circular wav fronts again set of all points that are in Phase with each other they're all crusting over here you can see another they're all cresting over here right so these are circular wav fronts but again if you look at the direction in which the wave is traveling look at that look at that you can see um locally at that particular point it is perpendicular to the wavefront it is perpendicular to the wavefront you can make sense that the wave is traveling outwards but wave direction is always perpendicular to the direction of the to the to the wfront that's an important one for us now to understand what happens when it enters a medium like glass let's just focus on one of these wave fronts and introduce our glass piece and what helps me to visualize this is instead of looking at this as a complete continuous wavefront let me convert it into a few dots representing the wave crests so you can imagine this is one of one of the wave crest of the wave this is another wave crest of the wave this whole thing is a wave front and now let's see what happens when it enters into the glass remember the waves slow down so let's see how that you know results in bending okay so let's look at it frame by frame so in the next frame all of them would have traveled equal distance forward because they're all traveling with the same speed again they will travel equal distance forward but now you can see that this one the one at the bottom this Crest enters the glass piece first therefore it's going to slow down first so in the next frame this one will travel less distance but the on over here would travel the same distance as before and see what happens as a result you'll get something like this that makes sense right this has traveled less distance but the rest of them have travel the same distance as before and now notice this one enters into the glass piece so now this one will slow down and so it will also travel less distance now this and this will travel at the same speed but the rest of them will continue traveling at the same um earlier speed now you see what happens boom can you see what's going on as a result notice what what do you notice oh my God you can actually now see the wavefront bending what caused the bending the bending happened because not all the wave crusts slow down at the same time the ones at the bottom slow down first and then slowly and steadily the rest of them start slowing down and now if you could see the wence just like before there you have it this is what it would look like let's animate this one more time here's what it would look like there it is the Ws are bending they've slowed down and they're bending because of that beautiful right but now remember if you want to figure out the direction in which the light Ray is traveling that direction will always be perpendicular to the direction of the wave front so let's look at one more time and focus on just a beam of light and let's see what we get so we're just going to focus on a beam of light and boom there you go you can now clearly see the whole thing bending beautiful right this is how we model refraction thinking of light as an electromagnetic wave okay let's test our understanding quickly what if the light did not slow down as much much as in glass for example if this was a different medium in which light was slightly faster what would this bending look like well now it wouldn't bend as much because the wavefront will not slow down as much as it did before and as a result look they won't Bend as much as they did before so you can see how much the light Ray bends clearly depends upon how much this light slows down how much the light changes its speed but another thing that would matter is the angle at which we incident light for example what if we incident light perpendicular to the surface what would happen then well now light wouldn't bend at all why because even though light is slowing down look the entire wavefront enters at exactly the same time and therefore the whole wavefront slows down together there's no bending so when you incident light perpendicularly there will be no bending at all so it depends on the angle at which you incident light as well this means the amount of bending depends upon the angle at which you shine light on the surface and it also depends upon how much light slows down or speeds up it can speed up as well and now that brings us to the second question more fundamental one why does light change its speed in the first place why does light for example slow down when it goes from say air to Glass what really happens to the electromagnetic waves when it enters a glass one of the cool ways to figuring this out is to instead of looking at the entire glass piece at once just take a tiny very tiny very thin portion of the glass and see what happens to it okay so if the glass piece was not there then the wave would just keep going forward but because the glass piece is there and the glass contains electrons inside of it we can model these electrons to be kind of like a tiny spring and so now when the electromagnetic waves go over those electrons let me just show that so here is an electron for example and as the electromagnetic wave passes over that electron look it makes it go up at down it forces it to go up and down and so we have these electrons that are all going up and down oscillating up and down and what happens when charges oscillate up and down they create their own electromagnetic waves ooh this means all the electrons of this glass piece are going to generate another electromagnetic wave they'll come out in both the direction I'm just going to show it over here so that we can see them apart so here it is now the one going to the left is the reflected light but since we're not interested on that just let's look at the one that's going to the right it's going to merge with the incoming light with the original light that we have incident and now the big question is what happens when they merge together well the technical term over here is interference we say that these two waves will interfere with each other and to figure out what happens to the resulting wave when waves interfere with each other all we have to do is just add up their heights at each location for example if we have a situation like what we have over here where the crests and the troughs the Peaks and the valleys are all lining up like this then in the resulting wave the Peaks would be even bigger The Valleys would be even deeper right they all add up and we would get something that looks like this we'll get a wave that looks very similar to this but bigger Peaks and bigger valys we call this constructive interference because they are constructing each other right right but what if the secondary wave was not like this what if the secondary wave was like this where the Peaks matched exactly with the valleys of the secondary wave what would happen then now they tend to cancel each other the Peaks over here tends to cancel with the valleys and The Valleys over here tends to cancel with the Peaks but because this wave is still much smaller compared to this one because this wave is generated by a tiny sliver of a glass piece over here so it's not much so the height of this wave should be very very tiny compared to the height of this one but what will end up happening the net result is that you'll get a wave that looks again very much like this but with decreased Peaks and valys we call this destructive interference because they're destroying each other in fact if this wave had the same height as this wave they would completely destroy each other because the Peaks would be completely destroyed by the valys the valys would be completely destroyed by the Peaks and so on which means to understand what really happens over here we need to know exactly what what is the phase difference between these two waves in other words we need to understand you know um how are the crest and the crest how are the crests of the waves and the troughs of the Waves aligned like how are they aligned over here now for that we need to do the math which we're not going to do but if we do the math it turns out for visible light frequency and for glass-like material we would find that the secondary waves are not the the Peaks are not completely lined up but they are slightly shifted back like this in fact the peak of this will line up with the zero of the secondary wheel something like this this how it they will line up unfortunately we will not be able to understand intuitively as to why it lines up that way because we have to do the math which we're going to skip over here but now comes the question what would the resulting wave look like well the resulting wave will not be a constructive interference nor a destructive interference it'll be something in between what we will end up seeing is a wave again that looks pretty much like this but since the peaks of these are not lined up the resulting one will have a peak that is somewhere in between the peak of this one and the peak of this one it'll be very close to the peak of this one but it'll look somewhat like this so you can kind of see in fact I think it should be very close like this okay kind of like this so the peaks of this one will be very very close to the peaks of this one but because of this wave it will be slightly pulled back so the net effect like putting it all together to summarize the net effect of this secondary waves is that the peaks of these waves get slightly pulled back so if I draw that wave over here it would look very similar to this but it would be slightly pulled back look at it carefully boom this is the effect of one tiny glass piece over here what if I put another glass piece somewhere over here well again the wave in front of it gets pulled back same idea boom what if I put another one pulls back so notice that the wave is not really slowing down the light is actually traveling at the same speed but when you introduce these glass pieces because of interference the wave gets pulled back so imagine a ball or something that's moving forward at some speed but then it gets pulled back and then again continues to travel at the same speed where it's pulled back now this is what happens if there are discrete tiny pieces of glass but remember we have a continuous medium which means there will be continuous pullback happening again imagine this ball going forward at some speed but being continuously pulled back what it would look like here it is and we'll compare it to the ball that is not being pulled back look at this can you see it both the balls are always traveling at the same speed but this one starts lagging because it's being continuously pulled back it almost looks like it is slower than the other Ball but it's not but it gives you that illusion of being slower the same thing is going to happen over here because of this continuous pull back it will give us the illusion that the light has slowed down and as a result you will now get get light traveling at a slower speed and you'll also see the veiling being compressed because of the pullback effect as well finally when you shine white light on a piece of glass since it has different colors meaning different frequencies they would have different pullback effects therefore different colors will travel with different speeds it turns out that the Viet end of the spectrum will have the maximum pullback effect and so it'll travel with the slowest speed but the red end of the spectrum will have the least pullback effect and it will travel with a higher speed which means if you shoot white light at an angle they won't bend the different colors will bend differently the wet since it slow down a lot will bend more compared to Red so red will bend less white will bend more and as a result and of course all the colors will be in between and as a result the colors separate out that's why the prism separates out all the colors and this is also the reason why we see Rainbow was