Question

Consider two polaroids $A$ and $B$ as shown. Unpolarised light is incident on polaroid $A$. Now, both the polaroids are rotated simultaneously by ${180}^{\circ }$ in same sense of rotation such that at every instant, their pass (transmission) axes always remain parallel to each other. During the rotation, intensity of transmitted light through polaroid $B$.

• A. increases continuously
• B. first increases then decreases
• C. remains same
• D. decreases continuously

Answer 1

Answer: (C)

remains same

Intensity after passing through polaroid $A$ is
$I_A=\frac{I_0}{2}$
$=\frac{\text{Intensity of unpolarised light}}{2}$
Intensity after passing through polaroid $B$ is
$I_B=I_A{\cos }^2\theta$
Here, $\theta =$ angle between pass axes of $A$ and $B$.
Here, pass (transmission) axes always remain parallel to each other i.e. $\theta =0$.
$\therefore I_B=I_A=\frac{I_0}{2}=constant$
Hence, during the rotation, intensity of transmitted light through polaroid $B$ remains same.