Question

Two polaroids are kept crossed to each other. If one of them is rotated an angle $60^{\circ}$, the percentage of incident light now transmitted through the system is

• A. 0.1
• B. 0.2
• C. 0.25
• D. 0.125

Answer 1

Answer: (D)

0.125

Answer 2

Let $I_{0}$ be the intensity of unpolarised light, so the intensity of first polaroid is $\frac{I_{0}}{2}$. On rotating through $60^{\circ}$ the intensity of lioht from second nolaroid is
$I = \left( \frac{I_0}{2} \right) ( \cos \, 60)^2$
$= \frac{I_0}{2} \frac{1}{4} = \frac{I_0}{8} = 0.125 I_0$
So, percentage of incident light transmitted through the system is $12.5\%$.