# Polarisation by reflection

Polaroid sunglasses will cut out the reflected glare from roads because the reflected beam is partly or totally polarised. This polarisation occurs when light is reflected from any non-conductor of electricity. Whether the polarisation is total depends on the surface and the angle of incidence.

For a particular angle p, the beam is completely plane-polarised, the reflected light being polarised as shown in Figure 5; p is known as the polarising angle for that material.

Brewster found that:

tan p = n

(known as Brewster's law)

where n is the refractive index of the material. It is simple to show that when light meets a surface at the polarising angle the reflected and refracted beams are at right angles to each other. Notice also that the reflected and refracted light has vibrations along the surface at the point of incidence. For glass with n = 1.54 the polarising angle = 57o.

Since n varies with the colour of the light, white light can never be perfectly polarised by reflection. The front of a glass door in a cupboard can show excellent polarisation by reflection.

The following diagram shows how we can use two mirrors as a polariser and analyser arrangement.