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Current time:0:00Total duration:11:06

Video transcript

if you look at the drawing of this dude through a glass slab then you see that his body appears to be shifted why do you think is that happening in this video we're going to focus on refraction through paddle sided media what's a paddle sided medium well let's take an example let's say we have a rectangular piece of glass a very long rectangular piece of glass you can say that the opposite sides of this piece are parallel to each other and so this is an example for paddle sided medium it doesn't have to be glass it can be any medium as long as it's opposite sides or its sides are parallel to each other so we're gonna look it'll focus on what happens when a ray of light falls on such a medium so let's say we have a source of light so you can imagine it's a bulb or a laser pointer or the Sun or something like that okay and so a ray of light let's consider one ray of light that starts from here and gets incident on the surface well we've already seen before that Ray the light changes its speed when it travels from one medium to another so for example over here is vacuum this is glass let's say so when light travels from vacuum to glass its slows down as an example and as a result of this change in speed it bends now we've seen that before and this bending is called refraction of light and so one of the ways to visualize this bending is we can just imagine that this source of light is shooting bullets we can imagine light is made of bullets so it is consider one such a bullet that's been shot out from this source and here it is here's that bullet now instead of a bullet I've drawn a car because it's I like cars it's easier to visualize this with a car and so as this bullet gets short forward it gets incident on the first surface of our glass piece and now if we can zoom in a little bit we will see that as the claw as the car enters this glass only one of its view has entered first and we've seen that Lights flows down in glass which means this wheel slows down but all the other wheels are pretty fast and as a result this car ends up bending this way because this wheel is faster than this wheel and if you could draw in normal we see that the car keeps bending to words the normal and it keeps bending until the entire car has entered the medium and then it keeps traveling in a straight line until it starts exiting the medium now again then it exits the medium you can see that this reel exits first so it gets faster and so now the car bends the other way again if you drew a normal we see that this time it bends ave away from the normal and again it keeps bending until the entire car exits the medium and then it follows a straight line and so the complete picture looks somewhat like this and here is a picture of light actually going through a piece of glass and you can pretty much see the same thing going on over here again notice these two sides are parallel to each other right now you may be wondering well what's the big deal why are we even talking about this well here's the reason why over here light bent towards the normal by some angle let's call that angle as something like I said 10 degrees as an example then then light exits notice it bends away from normal by exactly the same amount 10 degrees so over here it bent one way towards the normal by 10 degrees it bends here away from the normal by 10 degrees which means the tube endings cancel out because they're in the opposite direction one is clockwise one is anti-clockwise so overall the light has not suffered any bending at all and that's the speciality of bending or refraction through paddle sided media light does not suffer an overall bending so it comes back to its original direction now one question you may have is how do we know that these two bendings are exactly the same and one way to convince ourselves of this is thinking of it this way suppose the ray of light was shot in Reverse meaning the ray of light was incident over then this ray after refraction would travel like this and retrace the entire path backwards this is called the principle of reversibility and this is true because the rules of reflection or refraction don't depend on the direction in which the ray of light was incident so it wouldn't matter so by using this we can say that this ray of light when it exits glass must have deviated away or bent away by ten degrees away from the normal it must have because if we dint that it wouldn't be able to retrace that path and so from this we can say that when the ray of light when this ray of light exits glass it must Bend away by ten degrees and so by that logic over here also it must Bend away by ten degrees it's a profound profound argument and you can you know think about this for a while and as a result what we see is that there is no overall bending of light which means it's as if this piece of glass didn't even exist isn't it because there overall there is no bending so that's the speciality over here but of course you can see one one effect that the glass piece is having on the ray of light and that is this ray of light is a little shifted side words compared to this what I mean is if you were to back-trace this then notice this is parallel to this ray because we just discussed there is no overall bending but it doesn't appear to come from the same point it appears to come from somewhere over here and as a result we can now say that this ray is shifted a little bit so we can just draw well here this way we can say the ray of light has shifted by some amount to the side and this side would shift which is caused by this piece we give a name to it we call that as lateral shift hear the word lateral means side words lateral shift so the two important consequences of a parallel sided media is that one there is no overall bending of light and two the ray of light after exiting the medium is laterally shifted shifted side words a little bit compared to the incident ray and so now we can understand what's really going on over here over here when we're looking at this dude through that glass piece you can clearly see that the head is above the piece of glass which means we're looking at the head directly in other words the rays of light from the head is directly entering into the camera but the rays of light from the body enters through the glass before it enters into the camera and so if we could if we were to draw the situation let me just go back and let's say if we were to draw this situation let's draw a couple of ray diagrams for that situation so here is that same situation over here here is that glass piece something is in grade on that don't worry about this I couldn't find a clean piece of glass don't worry about that but if you were to draw a ray of light from the head then that ray of light is about this glass piece and so it goes straight into the camera but a ray of light from the body from some point onto the body first enters into the glass and thus it undergoes that lateral shift we just discussed which means when we look from here or when you put a camera over there so if you put a camera or an eye over there then this ray of light appears to come from somewhere over here and that's why his body appears to be disconnected now of course I have exaggerated the figure over here I've exaggerated the bending inside the glass but that's pretty much what's going on and that's why when you look from here the ray of his body appears to be little shifted as you can see all right before we wind up just a couple of questions to ponder upon if we come back to this picture here's a question what do you think will happen if we were to introduce one more parallel side media sides which are again parallel to these sides free field if we introduced it in the path of a viscera do you think that the emergent ray the Ray that comes out of that medium will still be parallel to the incident direction or do you think it will end up bending think about this for a while all right let's see if we introduce another medium completely different medium let's say but again paddle side it then the same thing must happen over here as well this reflect this ray of light must be in the same direction as this one because again the bending is canceled out but we already know that this ray is in the same direction as this one therefore this emergent ray must be in the same direction as this one in other words this ray must still be parallel so you see this is not limited to just one single medium even if you have multiple media as long as they are parallel sided we will see that the final emergent ray suffers no overall bending so if we were to draw a reference line we see that this final ray is still paddle to this initial incident ray and now this represents the total lateral shift that we are getting and another important thing we can see just from the diagram is that this lateral shift that we get is independent of the distance between the two media let me just show you that so if you move the second medium you will see that the emergent ray pretty much stays the same the shifted the shift doesn't change at all so even if we could take the second medium and connect it if the two mediums were joined together you can clearly see in even in this case the emergent ray will still be parallel to the incident ray so this is not limited to just one medium it has nothing to do it's nothing special about this single medium or this one remember this property that we discussed is a property due to the paddle sidedness of our media all right once super duper last thing which I want you to keep thinking about is what will happen to this amount of lateral shift if we were to change the thickness of the medium think about what will happen if you were to make this medium thinner or make it thicker think about this and how do you think lateral shift would change if we were to change the direction of the incident ray or maybe we were to turn this glass slab how do you think what what do you think would happen to the latter if do you think it would remain the same it would change think about this