What is polarisation of light? Describe an experiment with two Polaroids which shows that light is a transverse wave.

1. 1 · Define polarisation. Confining the vibrations of light to one plane containing the direction of travel.
2. 2 · Describe unpolarised light. Light from a bulb vibrates in all directions at right angles to the ray.
3. 3 · Use the polariser. The first Polaroid passes only vibrations along its axis, so the light becomes plane polarised.
4. 4 · Draw the set-up. [Source; unpolarised light; polariser P₁ with axis vertical; plane polarised light; analyser P₂ shown parallel (light passes) and crossed (no light)]
5. 5 · Turn the analyser. The light is brightest when the axes are parallel and zero when they are crossed at 90°.
6. 6 · Conclude. Light can be cut off this way only if its vibrations are at right angles to its direction of travel, so light is transverse.
7. 7 · Real life. Polaroid sunglasses cut glare from water and roads.

Scoring tip: Say clearly that a longitudinal wave could never be blocked by turning a Polaroid; that sentence proves the point.