8. Polarization of Light

Ordinary light (from the Sun, a bulb, or a candle) is described as 'unpolarised' because it consists of electromagnetic waves whose electric field vibrates in all possible directions perpendicular to the direction of propagation, with no preferred orientation. Polarization is the process of restricting these vibrations to a single plane, producing what is called plane-polarised light. This is possible only because light is a transverse wave (the vibrations are perpendicular to the direction of travel) — longitudinal waves such as sound cannot be polarised at all, since their vibrations already occur only along the direction of propagation.

Light can be polarised by passing it through certain crystals or specially manufactured sheets called polaroids (or Polaroid filters), which allow vibrations in only one particular plane to pass through and block (absorb) all others. If unpolarised light is passed through one polaroid and then through a second polaroid held at 90° to the first, no light emerges at all, because the second polaroid blocks the plane of vibration that the first one has already selected.

Sunlight scattered by the atmosphere (the same Rayleigh scattering that gives the sky its blue colour) is also found to be partially polarised, with the degree of polarization being greatest at 90° from the Sun's direction. Similarly, light reflected off a smooth, non-metallic surface such as water, glass, or a wet road is strongly polarised in the plane of the reflecting surface. This is precisely the glare that polarised sunglasses are designed to cut out, since their polaroid film is oriented to block light vibrating in that particular (mostly horizontal) plane while still allowing normally-scattered, unpolarised light from the rest of the scene to pass through, greatly improving visual comfort and contrast for activities such as driving or fishing.

Practical applications of polarization include polarised sunglasses (which cut down glare reflected off wet roads, water, or glass — such reflected light tends to be partially polarised), polarising filters used in photography to reduce reflections and enhance sky/cloud contrast, LCD (liquid crystal display) screens found in phones, calculators, and computer monitors, and 3-D cinema glasses, which use polarization to send a slightly different image to each eye.