Polarization of Light Video Lecture - Physics Class 12 - NEET

- Let's talk about polarization of light.
We know what light waves are;
they're electromagnetic waves.
So they're made out of electric fields.
And that's not good enough.
We know there's not just electric fields.
That couldn't sustain itself.
There's got to be magnetic
fields there, as well,
that are changing.
Those are perpendicular, so
you can kind of draw them.
It's hard, on something two-dimensional,
but you can kind of imagine those
looking something like this.
And those magnetic fields
would point at a right angle
to the electric fields.
But this gets really
messy if I try to draw
both the electric and magnetic
fields at the same time.
So we're going to leave
the magnetic fields out.
It's often good enough to
just know the direction
of the electric field when we
focus on the electric field.
So what does polarization mean?
Polarization refers to the
fact that, if this light ray
was heading straight toward
your eye, or a detector,
over here, what would you see?
Well, if I draw an axis over here,
and this point here, in the
middle, this is this line --
so imagine we're looking
straight down that line --
and then up and down is up and down,
and then left and right,
that direction I have the magnetic field,
would be this way and that way.
What would my eye see?
Well, my eye's only going
to see electric fields
that either point up or
electric fields that point down.
They might have different
values, but I'm only going to see
electric fields that point up or down.
Because of that, this
light ray is polarized.
So polarized light is light
where the electric field
is only oscillating in one direction.
Up or down, that's one
direction -- vertically.
Or it could be polarized horizontally.
Or it could be polarized diagonally.
But either way, you could
have this wave polarized
along any direction.
I mean, a light ray
like this, if we had it
coming in diagonal,
this light ray that's
oscillating like this,
where the electric field
oscillates like that,
that also polarized.
These are both polarized because
there's only one direction
that the electric field is oscillating in.
And you might thing, "Pff,
how could you ever have
"a light ray that's not polarized?"
Easy.
Most light that you get is not polarized.
That is to say, light
that's coming from the sun,
straight from the sun --
typically not polarized.
Light from a lightbulb, an
old incandescent light bulb,
this thing's hot.
You can get light
polarized in any direction,
all at once, all overlapping.
So if we draw this case for a light bulb,
just a random incandescent light bulb,
you might get light, some of
the light, hitting you eye,
you can get some light
that's got that direction,
you got light that's got this direction,
you got light in all these directions
at any given moment.
I mean, you'd have to add
these up to get the total,
and they might not all be the same value.
But what I'm trying to say
is, at any given moment,
you don't know what direction
the electric field's
going to be hitting your
eye at from a random source.
It could be in any direction.
So this is not polarized.
This diagram represents
light that is not polarized.
At some point, the field
might be pointing this way,
at some later point it's
this way; it's just random.
You never know which
way the electric field's
going to be pointing.
Whereas these over here,
these are polarized.
So how could you polarize this light?
Let's say you wanted
light that was polarized.
You were doing an experiment.
You needed polarized light.
Well, that's easy.
You can use what's called a polarizer.
And this is a material
that lets light through,
but it only lets light
through in one orientation,
so you're going to have a
polarizer that, for instance,
only lets through
vertically polarized light.
So this is a polarizer.
These are cheap:
thin, plastic, configured in a way so that
it only lets light through
that's vertically polarized.
Any light coming in here
that's not vertically polarized
gets blocked, or absorbed.
So what that means is, if
you used this polarizer
and held it in between your
eye and this light bulb,
you would only get this light.
All the rest of it would get blocked.
Or you could just rotate this thing
and imagine a polarizer
that only lets through
horizontal light.
Now it would only let through
light that was this way,
and so you would only get
this part of the light.
Or you could just orient
it at any angle you want
and block everything but the certain angle
that this polarizer is defined
as letting light through.
So you can do this.
And once you hold this up,
you get polarized light,
light that's only got one orientation.
So that's what polarization means.
But why do we care about polarization?
Well, let me get rid of this for a minute.
You've heard of polarized sunglasses.
So imagine you're standing near water,
or maybe you're standing on ice or snow
or something reflective.
There's a problem.
Say the sun's out.
It's shining.
It's a beautiful day -- except
there's going to be glare.
Let's say you're looking down at something
here on the ground.
It's going to get light
reflecting off of it from just ...
you know, light's coming
in from all direction.
But it also gets this
direct light from the sun.
So it gets light from
reflected off the clouds
and whatever, whatever's
nearby, ambient light.
And there's also this direct sunlight.
That's harsh.
If that reflects straight
up to your eye, that hurts.
You don't like that.
It blocks our vision.
It's hard to see, it's glare.
We don't want this glare.
So what can we do?
Well, it just so happens
that, when light reflects
off of a surface, even
though the light from the sun
is not polarized, once it
reflects, it does get polarized
or at least partially polarized.
So this surface here,
once this light reflects,
it's coming in at all orientations.
You got electric field ...
you never know what electric
field you're going to get
straight from the sun.
And when it reflects,
though, you mostly get,
upon reflection, the
direction of polarization
defined by the plane of
the surface that it hit.
So because the floor is horizontal,
when this light ray hits
the ground and reflects,
that reflected light
gets partially polarized.
This horizontal component
of the electric field
is going to be more present
than the other components.
Maybe not completely.
Sometimes it could be.
It could be completely polarized,
but often it's just partially polarized.
But that's pretty cool,
because now you know
what we can do.
I know how to block this.
We should get some sunglasses.
We put some sunglasses on
and we make our glasses
so that these are polarized.
And how do we want these polarized?
I want to get rid of the glare.
So what I do is, I make sure my sunglasses
only let through
vertically polarized light.
Here's some polarizers.
That way, a lot of this glare gets blocked
because it does not have
a vertical orientation,
it has a horizontal orientation.
And then we can block it.
So that's one good thing that
polarization does for us,
and understanding it,
we can get rid of glare.
Also, fishermen like it because,
if you're trying to look
in the water at fish, you want
to see in through the water,
you want to see this light
from the fish getting to you.
You don't want to see
the glare off of the sun
getting to you.
So polarized sunglasses are useful.
Also, we can play a trick on our eye,
if we really wanted to.
You could take one of these, make one eye
have a vertical orientation
for the polarization,
have the other eye with a horizontal ...
and you're thinking, "This is stupid.
"Why would you do this for?"
"This eye's going to get a lot of glare."
We wouldn't use these outside,
when you're, like, skiing or fishing,
but you could play a trick on your eyes
if you went to the movies and
you went and watched a movie.
Well, the reason our eyes see 3D is
because they're spaced a little bit apart.
They each get a different,
slightly different image.
That makes us see in 3D.
We can play the same trick on our eye
if we have the polarization like this.
If light, if some of the light
from the movie theater screen
is coming in with one polarization,
and the other light's coming
in with the other polarization,
we can send two different
images to our eyes
at the same time.
If you took these off,
it'd look like garbage
because you'd be getting both of these
slightly different images,
it'd look all blurry.
And it does.
If you take off your 3D
glasses and look at a 3D movie,
looks terrible, because now
both eyes are getting both images.
But if you put your glasses back on,
now this eye only gets the orientation
that it's supposed to get, and this eye
only gets the orientation
that it's supposed to get,
and you get a 3D image.
So it's useful in many ways.
Let me show you one more thing here.
Let's come back here.
This light was polarized vertically.
So that's called linear polarization.
Any time ...
Same with these.
These are all linear polarization
because, just up and down,
one linear direction,
just diagonal.
This is also linear.
All of these are linear.
You can get circular polarized light.
So if we come back to here,
we've got our electric field
pointing up, like that.
Now let's say we sent
in another light ray,
another light ray that
also had a polarization,
but not in this direction.
Let's say our other light
ray had polarization
in this direction, so it looks like this,
kind of like what our magnetic
field would have looked like.
But this is a completely
different light ray
with its own polarization
and its own magnetic field.
So we send this in.
What would happen?
Well, at this point, you'd
have a electric field
that points this way.
At this point, you'd have a electric field
that points that way.
What would your eye see
if you were over here?
Let's see.
If I draw our axis here.
All right, when this point
right here gets to your eye,
what am I going to see?
Well, I'm going to have a light ray
that's one part of a light ray.
One component points up.
That's this electric field.
One component points left.
That's this electric field.
So the total, my total electric
field, would point this way.
I could to the Pythagorean theorem
if I wanted to figure out the size of it,
but I just want to know
the direction for now.
And then it gets to here, and look at it:
they both have zero.
This light ray has zero electric field,
this one has zero electric fields.
So then it'd just be at zero.
Now what happens over here?
Well, I've got light.
This one points to the right
at that point, this pink one,
and then this red one
would be pointing down.
So what would I have at that point?
I'd have light that went this way,
and it would just be
doing this over and over.
It would just be ...
I'd just have diagonally polarized light.
This isn't giving me anything new.
You might think this is dumb.
Why do this?
Why send in two different waves
to just get diagonally polarized light?
I could have just sent in one wave
that was diagonally polarized
and got the same thing.
The reason is, if you
shift this purple wave,
this pink wave, by 90 degrees of phase,
by pi over two in phase,
something magical happens.
Let me show you what happens
here, if we move this to here.
Now we don't just get diagonally
linear polarized light.
What we're going to get is ...
Let me get rid of this.
Okay, so we start off with red, right?
The red electric field points up, and then
this pink wave's electric
field is zero at that point.
So this is all I have.
My total electric field would just be up.
I'm going to draw it right here.
The green'll be the total.
Now I come over to
here, and at this point,
there's some red electric
field that points up,
but there's some of this
other electric field
that points this way.
So I'd have a total electric field
that would point that way.
And then I get over to here,
and I'd have all of the electric
field from the pink one,
none from the red one.
It would point all left then.
Look what's happening.
The polarization of this
light, if I shift this,
if I'm sitting here, looking with my eye,
as my eye receives this light,
I'm going to see this light
rotate its polarization.
The polarization I'm going to notice
swings around in a circular pattern.
And because of this, we call
this circular polarization.
So this is another type of polarization,
where the actual angle of
polarization rotates smoothly
as this light ray enters your eye.
And you know what?
Er, drrr ...
All right, actually, I sent
you to receive this one first.
That makes no sense.
You're going to receive the
ones closes to you first
in this light ray going this way.
So you'd actually receive
this one first, then that one,
then this one, then this one.
Because of that, you wouldn't see this
going in a counterclockwise
way, you'd see this
going in a clockwise
circularly polarized way.
Sorry about that.
You might think, "Okay, why?
"Why even bother with
circular polarization?"
Well, I kind of lied earlier.
Turns out, in the movie theater example,
they don't actually do
it like this, typically.
Oftentimes in the movie theaters,
we don't have just linearly
polarized sunglasses.
This would be a problem
because, when you look
at the movie theater screen,
and if you were to tilt your
head just a little bit ...
Think about it.
This one's not really going to
get the right image anymore.
It's going to get some of both.
And this one's going to get some of both.
It's going to be blurry.
Your head would have to be
perfectly level the whole time,
which might be annoying.
So what we do is, instead,
we create circular polarized glasses,
so that this one would
only get one polarization,
this one would get the other direction.
This way, even if you tilt
your head a little bit ...
shoot, clockwise is clockwise,
counterclockwise is counterclockwise.
By using circular
polarization for 3D movies,
it can make it a little easier on you eyes
to see a better 3D image,
even if your head's tilted a little bit.