Question 1

What is the polarisation of light?

Accepted Answer

Light is a transverse electromagnetic wave: its electric field oscillates perpendicular to the direction of travel. Polarisation describes the orientation of that electric-field oscillation. In linearly polarised light the field stays in one plane; unpolarised light has fields pointing in all transverse directions.

Question 2

What does a polariser do?

Accepted Answer

A polariser has a transmission axis and only passes the component of the electric field aligned with that axis. Unpolarised light passing through it emerges linearly polarised along the axis, and its intensity drops to about half.

Question 3

What is Malus's law?

Accepted Answer

Malus's law gives the intensity transmitted by a second polariser (the analyser) set at angle θ to the first: I = I₀cos²θ, where I₀ is the intensity after the first polariser. At θ=0 all light passes; at θ=90° none does.

Question 4

Why do crossed polarisers block all light?

Accepted Answer

When two polarisers are crossed (θ=90°) the analyser's transmission axis is perpendicular to the incoming polarisation. The projected component is E₀cos90°=0, so cos²90°=0 and no light is transmitted.

Question 5

Why does the cos² appear in Malus's law?

Accepted Answer

The analyser passes only the field component along its axis: E = E₀cosθ. Intensity is proportional to the square of the field amplitude, so I ∝ E² = E₀²cos²θ, giving I = I₀cos²θ.

Question 6

What is a quarter-wave plate?

Accepted Answer

A quarter-wave plate is a birefringent crystal that retards one polarisation component by a quarter of a wavelength (90° phase). When linearly polarised light enters at 45° to its axes, the two components combine into circular polarisation, where the field vector rotates as the wave travels.

Question 7

What is circular polarisation?

Accepted Answer

In circularly polarised light the electric-field vector keeps a constant magnitude but rotates uniformly about the propagation direction, tracing a helix. It results from two perpendicular linear components of equal amplitude and a 90° phase difference.

Question 8

Why does the sky look polarised?

Accepted Answer

Sunlight scattered by air molecules (Rayleigh scattering) becomes partially polarised, strongest at 90° from the Sun. Polarising sunglasses and a rotated polariser reveal this by darkening parts of the sky.

Question 9

How do polarised sunglasses work?

Accepted Answer

Light reflected off horizontal surfaces such as water or roads is largely horizontally polarised. Polarised lenses have a vertical transmission axis, so they block this horizontal glare via Malus's law while still passing useful light.

Question 10

Does Malus's law conserve energy?

Accepted Answer

Yes. The light not transmitted is absorbed (or reflected) by the polariser material; it is not destroyed. The cos²θ factor describes only the fraction passed along the transmission axis, and the remaining energy is converted to heat in the polariser.

🕶️ Polarisation of Light — Malus's Law & Polarisers

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