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 sytem is

• A. $10\%$
• B. $20\%$
• C. $25\%$
• D. $12.5\%$

Answer 1

Answer: (D)

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