Polarisation

Polarisation

In a wave, there is always something that oscillates. This can be each point on a rope wave, the air molecules in a sound wave, or the electric field in light. If this oscillation is perpendicular to the velocity of the wave, the wave is called a transverse wave. If the oscillation is parallel to the velocity of the wave, the wave is a longitudinal wave.

A rope wave is a transverse wave. The wave travels along the rope, while each point on the rope oscillates perpendicular to the rope. Suppose that the rope is horizontal. You are holding one end of the rope, with the rope fixed to a wall in front of you. If you shake the rope up and down, you make a wave on the rope. This wave travels away from you, while each point on the rope moves up and down. So this oscillation is perpendicular to the wave velocity.

Now shake the rope left and right. You again create a wave that moves away from you. This time, each point on the rope moves left and right. The direction of oscillation is again perpendicular to the rope, and the wave velocity.

You have just succeeded in making the rope oscillate in two different directions - up-down, and left-right. In either case, the oscillation is perpendicular to the wave velocity - that is, the wave is transverse.

This suggests that in a transverse wave, there is more than one possible direction of oscillation. In fact, there are many. You can also shake the rope in a slanted direction, at any angle you like. We call this direction of oscillation the polarisation of the wave.

Next, we look at a longitudinal wave. You are standing at one end of a long spiral spring on a table. The other end of the spring is fixed at the opposite edge of the table, directly ahead of you. Hold your end of the spring. Push and pull quickly in the direction of the spring. This compresses and stretches the spring repeatedly, and creates a longitudinal wave that travels along the spring. The oscillation of each point on the spring is along the axis of the spring, which is also the direction of the wave velocity. Can you also shake the spring up-down or left-right?

Of course you can, but then you would make a transverse wave. So there is only one possible direction of oscillation in a longitudinal wave - along the direction of the wave.

It looks like we only have the freedom to change the direction of oscillation in a transverse wave, but not in a longitudinal wave. We often say that a transverse wave can be polarised, whereas a longitudinal wave cannot.

It is this idea of polarisation that provides one evidence that light is a transverse wave (and therefore a wave). But we cannot see the oscillating electric and magnetic fields in light, so how do we know whether light can be polarised?

You may have heard of polaroid sun glasses. This is a type of sun glasses, with lenses made of a special material that can remove the glare from sea water. If you have been to the beach on a sunny day, you have seen the very bright reflection of sunlight from the sea. Normal sun glasses would reduce the glare, but polaroid sun glasses would be able to remove nearly all of it. How does it work?

Imagine that you are at the beach on a sunny day, wearing a pair of polaroid sun glasses and looking the the sea comfortably. Now, take off the sun glasses. Cover one eye, and look through one of the lenses with your other eye. Slowly rotate the sun glasses, in such a way that you could continue to look through the lense. You would see the glare growing. When you have rotated 90°, you would be blinded - all the glare has come back. What happened?

When you rotate the sun glasses, the lense rotates. When the sun glasses are horizontal, the glare is blocked. When the lense is rotated by 90°, the glare goes through. This suggests that there is something in the light that has direction. This thing must be perpendicular to the light velocity. Otherwise, rotating the lense would make no difference.

So if light is a transverse wave, this could explain the above behaviour of the sun glasses. Of course, the above observation alone does not prove that light is a transverse wave, but it is very suggestive.

There is something special about the glare from the sea. This is light reflected from the water. The electric field in sunlight oscillates in many different directions that are perpendicular to the direction of the light. We say that sunlight is unpolarised. When it reaches the sea water, the reflected light consists mainly of oscillations in one direction only. The rest of the light goes into the sea. We say that the reflected light is polarised. The lenses in the polaroid sun glasses allow only light with one polarisation (oscillation direction) to go through. The polarisation of each lense is perpendicular to the polarisation of the glare. So the glare gets blocked.

The Physics Notes