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SHAOUL EZEKIEL: In this demonstration,
we're going to show a simple way of rotating
the plane of polarization of laser light from zero degrees
to anywhere to 90 degrees using simple techniques, for example,
by the use of polarizers.
The setup that we're going to use is here.
We have a laser, helium neon laser.
And here is the beam of the laser light.
We're going to reflect it off this mirror
and then reflect it again off this second mirror.
And then we'll go through a polarizer.
And then the light coming out of the polarizer
will go onto the screen.
The purpose of this polarizer is to establish
the state of polarization of the light coming out
of the polarizer.
If the polarizer is any good, the state
of polarization after the polarizer
should be plane polarized.
Now the little arrow here labels the transmission axis
of the polarizer, which means that the state of polarization
of the light then is aligned with that arrow.
Now let us show, indeed, that the state of polarization
of that polarizer is indeed as we said it was.
So we take another polarizer.
And we put it over here.
Again, the arrow labels the transmission axis
of this second polarizer.
Now, when I have the arrow vertical,
the same as in this case, the light is around its peak value.
It's very difficult for us to see
what's happening around the peak value.
So it's nice to turn the transmission
axis of this polarizer until we extinguish the light, which
means that the transmission axis of this polarizer,
which is going to be referred to as the analyzer,
is orthogonal to the axis here.
And the fact, as we look on the screen, the fact
that we can null the light means that the polarization
here before entering this analyzer
is, indeed, plane polarized.
And the direction of the arrow here,
which labels the null intensity, which
means that the transmission axis now of the polarizer
is orthogonal to the polarization of the light,
is then indicated by the direction of the arrow.
But we all know that the true polarization of the light
is in the vertical direction.
But it's much easier for us to label
the block axis, the direction of the block axis.
So please keep in mind that even though I
might show you the null on the screen,
the actual polarization is 90 degrees away from that.
Now I'm going to demonstrate the rotation
of the plane of polarization by means of the polarizer.
First, what I'm going to do is set this analyzer back
to where it was, set the arrow vertical,
and then take another polarizer, then,
to rotate the plane of polarization.
Now, at the beginning, I'm going to set this polarizer
arrow, which again labels the transmission
axis of this polarizer, in line with all the others.
Then I want to rotate the plane of polarization of the light
by this polarizer, let's say by 45 degrees.
Well, in order to check that, indeed, I
have rotated by 45 degrees, again I'm
going to track the null.
So now if we look on the screen, and I rotate the analyzer,
let's see where null is this time.
And here it is.
We can see that null is when the arrow is
in this direction, which is, again, 90 degrees
to the position in this direction, which
is about 50 degrees or so from where it was before.
Now let me go back to aligning the analyzer
with the peak transmission.
And you can see that the peak transmission--
it's not that difficult to see.
But you can see that it is about half or so,
about half the intensity of the peak transmission that I
had when this polarizer was in the vertical direction.
Now, if I want to rotate the polarization some more,
let's say close to 90 degrees, again
track the direction of the null, now, as we watch on the screen
here, the position of null now is over here.
And it shows that the arrow is pretty close to vertical, which
means, again, we've rotated the plane of polarization
some more.
Now, the interesting thing about going close to 90 degrees
is that the peak transmission, the peak transmission
of the light, as you can see over here, is not so big.
And, in fact, if I go even closer to 90 degrees,
I don't even get any light transmitted
through the polarizer.
So the conclusion, then, is that I
can rotate the plane of polarization of light
using a simple polarizer.
But the penalty is that as I get close to 90
degrees of polarization rotation,
I get very little light transmitted.
So in order to overcome this problem,
we use a second polarizer.
And we place the second polarizer.
In this case, I'll place it before the first,
before this polarizer here.
And remember, even before I put it in,
that the polarization of the light
now is along this axis, which is 90 degrees with respect
to this one.
And that's why we didn't get any light through the analyzer.
Now, I'm going to put this polarizer here.
And this polarizer is still along this direction, which
is orthogonal to this one.
And before, remember, there was no light coming through.
And now you can see by adjusting the axis of this polarizer
here, I can get some light to come through
with polarization that is orthogonal to the original one.
And again, to go demonstrate that the null, now,
is along the vertical direction, which
means I have achieved 90 degree rotation using
now two polarizers and still have light left.
The sad story is that you only get
one quarter of the light coming out when you rotate
the polarization by 90 degrees.
But a quarter of the light at 90 degree rotation
is better than no light at all.
So to summarize, if you use only one polarizer,
you can rotate the state of polarization
by, let's say up to about 45 degrees
or so and only lose half the intensity.
If you want to go more, then you pay a penalty in intensity.
If you use two polarizers, then you can rotate any way
you want.
And the worst thing you can do, at 90 degree rotation,
is that you get only a quarter of the light coming through.