Question

Find an expression for intensity of transmitted light when a polaroid sheet is rotated between two crossed polaroids. In which position of the polaroid sheet will the transmitted intensity be maximum?

Answer 1

Let us consider three polaroids A, B and C where C is kept in between A and B.

Using Malu’s Law, the intensity of light emerging from the middle polaroid C is given by,

\(I_1 = I_o cos^2 \theta ; I_O\) is the intensity of light falling on middle polaroid.

Therefore, intensity  falls on the polaroid at the end (polaroid B) whose polarisation axis makes an angle of (90° - \(\theta\)) with the polarisation axis of the angle of middle polaroid.

Therefore, Intensity of light emerging from Polaroid B is, 

That is, transmitted intensity will be maximum when sin 2\(\theta\) = 1

2\(\theta\) = 90°

\(\theta\) = 45°