Polarisation

Polarisation is the restriction of the oscillations of a transverse wave to a single plane.

Unpolarised vs polarised light

Unpolarised light: oscillates in all planes perpendicular to the direction of travel. Light from the Sun, bulbs and candles is unpolarised.

Polarised light: oscillates in only one plane. Created by passing unpolarised light through a polarising filter.

Demonstrating polarisation

1. Pass unpolarised light through a polarising filter (the polariser). The transmitted light is polarised in one plane.
2. Place a second filter (the analyser) in the path and rotate it.
3. When the analyser is aligned with the polariser, maximum light passes through.
4. When the analyser is at 90$^\circ$ to the polariser, no light passes through.

This proves light is a transverse wave. If light were longitudinal, rotating the analyser would have no effect.

Applications

Polarised sunglasses: Reduce glare from reflective surfaces (water, roads, snow). Light reflected from flat surfaces is partially polarised horizontally. Polarised lenses block horizontal oscillations, reducing glare.

TV and radio aerials: Must be aligned with the polarisation of the transmitted signal (horizontal or vertical) to receive maximum signal strength.

3D cinema glasses: Each lens transmits a different polarisation, so each eye receives a slightly different image.

Common errors and how to correct them

• Saying sound can be polarised. Only transverse waves can be polarised; sound is longitudinal.
• Confusing filtering (reducing intensity) with polarisation (restricting the plane of oscillation).

How to apply this in an exam

Explain that polarisation restricts oscillations to one plane. Describe the two-filter experiment. State that only transverse waves can be polarised and explain why this proves light is transverse.