Question

Plane polarised light is passed through an analyser and the intensity of emerging light is reduced by $75$%. Optical vibrations make an angle $\theta$ with the axis of analyser. Then $\theta$ is.

• A. ${60}^{\circ }$
• B. ${45}^{\circ }$
• C. ${50}^{\circ }$
• D. ${58}^{\circ }$

Answer 1

The correct option is A ${60}^{\circ }$

Using the Malus' Law, the intensity of light after passing through a polarizer is given by $I=I_0{\cos }^2\alpha$, where $\alpha$ is the angle between light's polarization direction and axis of the polarizer.

Given $\frac{I}{I_0}=0.75\Rightarrow {\cos }^2\alpha =0.75\Rightarrow \alpha ={30}^o$

But, the optical vibrations are perpendicular to the polarization axis. Thus $\theta +\alpha ={90}^o$

$\therefore \theta ={60}^o$