Polarisation

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Electromagnetic waves

All electromagnetic waves are traverse waves.

A transverse wave is one in which the vibration is perpendicular to it's direction of travel.

Transverse waves can be shown on a graph in 2 ways:

  • Displacement against time
  • Displacement against distance

These two graphs have the same shape.

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Polarised waves

Analogy:

Imagine shaking a rope. You can shake it up and down, side to side, or diagonal to diagonal - it can go in any direction and it will still be a transverse wave.

But imagine shaking a rope through a vertical fence. The rope can then only create waves if the vibrations are vertical (up and down). The fence removes vibration in other directions. The rope (wave) has been polarised.

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Electromagnetic polarisation

Electromagnetic radiation is made up of two transverse waves (electric and magnetic fields).

Light that passes through a polarising filter can only vibrate in one direction.

If you have two polarising filters at right angles to each other, then no light can get through. This is because all directions of vibration will be blocked.

Polarisation can only happen to transverse waves.

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Polarising light

Rotating a polarising filter in a beam of light shows the fraction of the light that is vibrating in each direction.

If a beam of unpolarised light is direted at a reflective surface and the reflected ray is viewed through a polarising filter, the intensity of light leaing the filter changes with the orientation of the filter.

This is because light is partially polarised when it is reflected.

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Uses of polarisation

TV aerials

The rods on TV aerials ar always horizontal. This is because TV signals are polarised by the orientation of the rods o the broadcasting aerial. To recieve a strong signla, the receiving aerial has to have rods that are lined up with the rods on the transmitting aerial.

Communication satellites

These use different polarisations for signals in the same frequency band to reduce the amoutn fo interference between the signals.

CD players

A polarising crystal is used to transmit light polarised in one direction, and reflect light polarised in any other direction.

Photography

Polarising lenses are used to remove unwanted reflections in pictures taken.

Nature

Bees use polarised light to navigate, so they can direct other bees back at the hive to a good pollen spot they found.

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