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

- 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 parallel sided media. What's a parallel 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 parallel sided medium. It doesn't have to be
glass, it can be any medium as long as its opposite
sides, or its sides, are parallel to each other. So we're gonna 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 have already seen
before that ray, that 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, it
slows down, as an example. And, as a result of this
change in speed, it bends. Now, we have 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 we'll just consider one such bullet that's been shot out from this source. And here it is, here is 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 shot 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 car enters this glass, only one of its wheel has entered first. And we have seen that
light slows 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 a normal, we see that the car keeps
bending towards 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, when it exits
the medium, you can see that this wheel exits first. So it gets faster, and so now
the car bends the other way. Again if we drew a normal,
we see that this time it bends away, 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 is the reason why. Over here, light bend towards
the normal by some angle, let's call that angle as, I
don't know, something like let's say 10 degrees as an example. Then when light exits, notice
it bends away from the 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 two bendings cancel out because they are 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, parallel 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 here. 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 routes 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 10 degrees
away from the normal. It must have, because it it
didn't, then it wouldn't be able to retrace that path. And so from this, we can say
that when the ray of light, that this ray of light exits glass, it must bend away by 10 degrees. And so, by that logic, over
here also, it must bend away by 10 degrees. It's a profound, profound
argument, and you can, you know, think about this for awhile. And as a result, what we
see 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 that 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 one the ray of light. And that is this ray of
light is a little shifted sidewards compared to this. What I mean is if you
were to back trace this, then notice, this is parallel to this way 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 way is shifted a little bit. So we can just draw over here this way, and we can say the ray of light
has shifted by some amount, to the side. And this sidewards shift,
which is caused by this piece, we give a name to it. We call that as Lateral Shift. Here, the word 'lateral' means sidewards. 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 sidewards 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 are
looking at this dude, through that glass piece,
you can clearly see that the head is above the piece of glass. Which means we are 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 are going
to draw this situation, let me just go back, and
let's say 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 engraved on that, don't worry about this,
I couldn't find a clean piece of glass, don't worry about that. But, if we were to draw a
ray of light from the head, and that ray of light is
above 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 a 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 shifted, as you can see. Alright, 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 sided medium? Sides which are, again,
parallel to these sides. If we introduce it in
the path of this way, 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'll end up bending? Think about this for awhile. Alright, let's see. If we introduce another medium, completely different medium let's say, but again, parallel sided. Then, the same thing must
happen over here as well. This ray of light must be in
the same direction as this one because again, the bendings cancel 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 parallel 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 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
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 parallel
sidedness of our media. All right one super duper last thing which I want you to keep thinking about, is what'll happen to this
amount of lateral shift if we were to change the
thickness of the medium? Think about what'll 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
do you think would happen to the lateral shift? Do you think it would remain
the same, it would change? Think about this.