Question

Unpolarised light of intensity $I$ passes through an ideal polariser A. Another identical polariser B is placed behind A. The intensity of light beyond B is found to be $\frac{I}{2}$. Now another identical polariser C is placed between A nd B. The intensity beyond B is now found to be $\frac{I}{8}$. The angle in degrees between polariser A and C is –

Answer 1

Polaroids A and B are oriented with parallel pass axis. Let polaroid C be at angle θ with A, then it makes θ with B also.
$\frac{I}{8}=\left(\frac{I}{2}{\cos }^2\theta \right)\times {\cos }^2\theta$
${\cos }^2\theta =\frac{1}{2}$
$\theta ={45}^{\circ }$

Why this question? Concept: For a polarised light the entire intensity passes through a polariser, if pass axis lies in the plane of polarisation of light.