Illustrated Physics
physics, drawn
Concept

Polariser — where it appears

A device that transmits light whose field lies along one direction and removes the component across it. Sheet polarisers absorb that component with aligned molecular chains, wire grids reflect it, and crystal prisms split the two polarisations apart by refraction.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

The pattern the whole sky is written in. The sky as a disc — zenith at the centre, horizon at the rim, equal angles at equal distances — with the sun 30° above the horizon. Each short line is the direction the electric field vibrates in at that point, and its length and darkness are how polarised the light there is. The directions are perpendicular to the plane containing the sun, the observer and the point, which puts them tangent to circles centred on the sun. The heavy arc is the locus 90° from the sun, where the polarisation is strongest — 74 per cent here — and it is a great circle rather than a patch: a band across the sky, not a region near the horizon. This is what a polarising filter on a camera acts on, and it is why turning one darkens a band of sky and leaves the rest almost untouched, and why the effect is strongest when the sun is off to one side and absent when it is behind the photographer.

The pattern the sky is written in

Scattered sunlight is polarised, so the whole sky carries a direction of vibration at every point — arranged in circles about the sun, strongest on the great circle ninety degrees away from it, and vanishing at points that were found by looking before anyone could explain them. Bees navigate by it and a camera filter reads one band of it.

optics · Polarisation How fine a grid has to be to polarise. The fraction of the power transmitted by a grid of thin parallel wires, against the ratio of the wire spacing d to the wavelength, on logarithmic axes — for the field along the wires with three wire radii (solid), and for the field across the wires with the thickest (dashed), from the quasi-static theory of strip gratings, valid while the spacing is under about half a wavelength. Across the wires nearly everything passes at every spacing. Along them the leak is about four times the square of (d/λ)·ln(d/2πa): 0.1% of the power at a spacing of a hundredth of a wavelength, 10% at a tenth, 64% at four-tenths, for wires of radius 3% of their spacing. Thicker wires block better because the inductance of the grid falls. The diamond marks Hertz's grid of 1888, 4% leaking along the wires; a visible-light polariser needs the same ratio at a wavelength of half a micrometre, which means wires about 140 nanometres apart.

The grid that stops the light along its wires

A row of parallel wires looks like a fence, and a fence ought to let through a wave that shakes along its slats. A grid of wires does the opposite. A field along the wires drives electrons up and down them, the currents re-radiate, and the wave is reflected; a field across the wires can move charge only through a wire's width, and passes. Heinrich Hertz used such a grid in 1888 to show that his new waves were polarised. The same grid, shrunk two hundred thousand times to wires 140 nanometres apart, polarises visible light in projectors — and every sheet of polarising plastic is the same idea made from chains of iodine.

optics · Polarisation

Named alongside it

The objects these essays reach for when they reach for this one.

PolarisationAtmosphereDegree of polarisationDepolarisationDipole radiationExtinction ratioThe Faraday cageMeasurementMultiple scatteringNavigationRayleigh scatteringReflective polariser

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