3.1.5

# Polarisation

Test yourself

## Polarised Light

Polarised light only vibrates in one plane.

### Polarisation direction

• Imagine holding a rope and shaking it so it forms a transverse wave.
• If you shake the rope from left to right (horizontally) you will form waves that are horizontally-plane polarised.
• This is because the wave lies in the horizontal plane.

### Polarisation direction 2

• If you shake the rope up and down (vertically) you will form waves that are vertically-plane polarised.
• This is because the wave lies in the vertical plane.
• These are two possible polarisation directions.
• You can have a whole range of polarisation directions: all the angles from 0° to 360°, not just left-right and up-down.

### Evidence for the transverse nature of waves

• Longitudinal waves cannot be polarised.
• If a wave can be polarised, that must mean that the wave is transverse.

## Applications of Polarisations

Polarisers only allow transverse waves at particular angles to pass through. This is useful for sunglasses and television transmission and reception.

### Polarisers

• Polarisers only allow transverse waves at particular angles to pass through.

### Polaroid material

• An example of a polariser is Polaroid material. This is used in sunglasses.
• Sunlight is unpolarised.
• The vibrations in the electric and magnetic fields are at all polarisation directions possible.
• If we saw a beam head-on we would see all the polarisation directions.

### Polaroid material 2

• Polaroid sheets are made of long molecules of quinine iodosulphate.
• The molecules run up and down the sheet.
• Any polarisation direction that is parallel to the molecules is absorbed.
• Any polarisation direction that is perpendicular to the molecules is transmitted.
• The transmitted light is at a lower intensity.

### Reflection as polarisation

• When unpolarised light reflects off a transparent surface, the light becomes polarised.
• The polarisation is partial and parallel to the surface.

### Polarisers at right angles

• If two polarisers, like Polaroid sheets, were placed at right angles to each other, no light would be able to get through.
• The first Polaroid would transmit only horizontally polarised light. But the second Polaroid can only transmit vertically polarised light.
• The incoming horizontally polarised light will be completely absorbed by the second Polaroid.

### Aerials for transmission and reception

• A television or radio aerial can either be orientated so its spokes are vertical or horizontal.
• If a transmitter sends out a vertically polarised electromagnetic wave, the spokes on the aerial must also be aligned vertically.
• This is because maximum absorption of the vertically polarised wave is needed to get a good signal.