Light from an unpolarised source is allowed to fall on a piece of Polaroid P. This plane-polarises the light (it is therefore known as the polariser), which then falls on a second piece of Polaroid A. This second piece is called the analyser because it is used to analyse the polarisation of the beam from P (see Figure 1).
As A is rotated the intensity or the light emerging from it
slowly changes, being a maximum at one point and a minimum after a further 90o
rotation. Rotating A by another 90o will bring back a maximum once more. A theory for
this was proposed by Malus in 1810.
Assume that light of amplitude Eo is incident
on a piece of Polaroid which has its plane of polarisation at an angle to the direction of polarisation
of the light.
Let the transmitted amplitude be E. Now E = Eo cos θ, and since the intensity I is proportional to the square of the amplitude we
have: