Polarisation of light: WACE Year 12 Physics Unit 4

What this dot point is asking

WACE wants you to explain what polarisation is, how filters produce and detect it, and why it proves light is transverse. This is the one wave property that distinguishes transverse from longitudinal waves.

Transverse waves and polarisation

In a transverse wave the oscillation is perpendicular to the direction of travel, so it has a definite orientation in the plane across the beam. Unpolarised light from a typical source contains electric-field oscillations in every direction perpendicular to travel, mixed randomly. Polarised light has its oscillation confined to a single direction.

How a polarising filter works

A polarising filter has a transmission axis and passes only the component of the electric field aligned with that axis, absorbing the perpendicular component. Sending unpolarised light through one filter therefore produces polarised light and roughly halves the intensity, because on average half the oscillation lies along the axis.

Crossed filters

If polarised light then meets a second filter (an analyser) whose axis is parallel to the first, it passes through. If the second axis is at right angles to the first, no component of the already-polarised light lies along it, so the light is completely blocked. Rotating one filter between these positions smoothly dims the light from bright to dark, which is the standard demonstration that the light has been polarised.

Why only transverse waves can be polarised

A longitudinal wave, such as sound, oscillates along its direction of travel and has no sideways orientation to select, so it cannot be polarised. The fact that light can be polarised therefore shows it must be transverse. This was important historical evidence pinning down the nature of light waves.

Everyday applications

Polarising sunglasses have a vertical transmission axis that blocks the strongly horizontal glare reflected off roads and water. Photographers use polarising filters to deepen sky contrast and cut reflections, and stress patterns in transparent plastics show up between crossed polarisers because the material rotates the plane of polarisation.

Malus's law and partial angles

For angles between parallel and crossed, the transmitted intensity follows Malus's law: if already-polarised light of intensity $I_1$ meets a filter whose axis is at angle $\theta$ to the light's polarisation, the transmitted intensity is $I=I_1\cos^2\theta$. This gives the full range smoothly: at $\theta=0$ the factor $\cos^2 0=1$ and all the light passes; at $\theta=45^\circ$ the factor is $0.5$ so half passes; at $\theta=90^\circ$ the factor is $0$ and the light is extinguished. Remember the first filter acting on unpolarised light is a special case: it always transmits half, regardless of orientation, because the source contains all directions equally. Only once the light is polarised does the $\cos^2\theta$ rule apply to subsequent filters.

Explaining the evidence

When asked why polarisation matters, state clearly that it can occur only for transverse waves and therefore shows light is transverse. Describe the two-filter test, noting that the second filter dims the light as it rotates and blocks it completely when crossed.