Polarisation

Polarisation (polarization in the US) means making a wave oscillate in one direction only. Light from the Sun or a bulb is unpolarised: it oscillates in every direction at right angles to the way it travels. Pass it through a polarising filter and only one direction gets through, giving plane-polarised light. Only transverse waves can be polarised, so polarisation is evidence that light is a transverse wave.

Polarised and unpolarised waves

OpenStax describes polarisation as the attribute that a wave's oscillations have a definite direction. For light, that direction is the direction of the electric field.

• Unpolarised waves oscillate in many directions, all square-on to their path. Light from the Sun is unpolarised.
• Plane-polarised waves oscillate in one plane only, for example only up and down.

Think of a rope tied to a wall and passed through a fence of vertical slats. Shake it up and down and the wave passes through; shake it side to side and the slats stop it. The slats act like a polarising filter.

Why only transverse waves can be polarised

A transverse wave vibrates across its path, and across can mean up and down, side to side or any angle between, and a filter can select one. In a longitudinal wave, such as sound, the oscillations are along the direction of travel, so there is only one possible direction and nothing to filter out. AQA's A-level specification lists polarisation as evidence for the nature of transverse waves: the fact that light can be polarised shows it is transverse.

Polarising filters and crossed polarisers

A polarising filter, such as Polaroid, only passes the part of the wave that oscillates along its axis. Unpolarised light that passes through one filter comes out plane-polarised, and OpenStax notes that its intensity falls by a factor of 2.

Add a second filter and rotate it:

• With its axis parallel to the first, the polarised light passes through.
• As it turns, less and less light gets through.
• With its axis at 90° to the first (crossed polarisers), no light passes the second filter.

Malus's law

AQA's specification doesn't require it, but the intensity through the second filter follows Malus's law, where θ is the angle between the light's polarisation and the filter's axis:

For example, unpolarised light of intensity 80 W m⁻² falls to 40 W m⁻² through the first filter. If the second filter is at 60° to the first, the intensity becomes 40 × cos²60° = 40 × 0.25 = 10 W m⁻².

Uses of polarisation

AQA names two applications of polarisers: Polaroid material, and the alignment of aerials for transmission and reception.

Polaroid sunglasses and glare

Light reflected from water, glass or a road becomes partly polarised, mostly horizontally. Polaroid sunglasses have filters with vertical axes, so they block much of this reflected glare while still letting through other light. Photographers use polarising filters on cameras for the same reason.

Aligning aerials

Radio and television signals are transmitted plane-polarised, because the transmitting aerial makes the waves oscillate in one plane. A receiving aerial picks up the strongest signal when it is aligned with that plane, so television aerials are mounted either horizontally or vertically to match the transmitter.

Common mistakes

• Saying sound can be polarised. Sound is longitudinal, so it can't.
• Saying a filter "turns" the light. It removes the parts not along its axis; it doesn't rotate the oscillations.
• Forgetting the first filter halves unpolarised light. Use I₀ ÷ 2 before applying Malus's law at the second filter.
• Thinking crossed polarisers let a little through. Ideal crossed polarisers block all the light.
• Confusing polarisation with refraction or diffraction. Polarisation is about the direction of oscillation, not the direction of travel.

Light and the electromagnetic spectrum are covered in the Light and Energy course, and transverse and longitudinal waves explains the two wave types. For practice in a short timed paper, try today's A-level Physics: Year 1 paper.

Read next

Physics · Waves and lightIs light a wave or a particle? Wave-particle dualityIs light a wave or a particle? The evidence for each, from the double-slit experiment to the photoelectric effect, photon energy E = hf and duality.