Polarisation

Polarisation is when we take an unpolarised transverse wave and ‘filter’ it such that the oscillations (vibrations) only occur in a single plane.

This can be useful, see uses of polarisation.

How do we polarise waves?

Unpolarised waves have oscillations occurring in all sorts of directions. We only want to ‘let through’ one of those directions, so we need some way of filtering out the other directions.

To do that, it’s actually quite simple - we just create a ‘slot’ for the wave that is oscillating in the desired direction to pass through, while blocking the other directions.

For example, if we want to polarise a wave so that the oscillations occur in the vertical direction, we can create a vertical ‘slot’ for the wave to pass through. Then, waves oscillating in the horizontal direction (or any other direction) will be blocked.

What waves can be polarised?

We can only polarise transverse waves - not longitudinal waves.

That’s because the wave oscillations of transverse waves can be fixed to a plane (direction) - but longitudinal waves can’t.

Polaroid

Most waves aren’t exactly that big though, so we can’t just cut a slot into a piece of cardboard or something and expect it to work. Instead, we use a special material called polaroid.

It’s made up of long-chain molecules that are all aligned in the same direction. When an unpolarised wave passes through the polaroid, only the component of the wave that is oscillating in the same direction as the molecules is allowed to pass through - the rest is absorbed by the material.

For example, if the polaroid is aligned vertically, only the vertical component of the wave will pass through, while the horizontal component will be absorbed.

This essentially creates a ‘slot’ for the wave to pass through, just on a much smaller scale.

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