Polarisation
Physics: AQA A Level, topic 3.1.5
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.
Test yourself on Polarisation
Which of these statements are true about polarised waves?
If you shake a rope up and down, in what direction will the wave through the rope be polarised?
Which statements about polarised light are true?
If two identical polarisers are placed at right angles to each other what would happen?