Question

A plane polarized light is incident on a polariser with its pass axis making angle $\theta$ with $x$-axis, as shown in the figure. At four different values of $\theta ,\theta =8^{\circ },38^{\circ },188^{\circ }$ and $218^{\circ }$, the observed intensities are same. What is the angle between the direction of polarization and x-axis ?

• A. $98^{\circ }$
• B. $128^{\circ }$
• C. $203^{\circ }$
• D. $45^{\circ }$

Answer 1

Answer: (C)

According to Malus law, we have
$I=I_0{\cos }^2\theta \Rightarrow \frac{I}{I_0}={\cos }^2\theta$
Since the observed intensities are same for all values of $\theta$,
therefore, for the angles given in options, the observed intensity
should be same as well for all $\theta ,\theta =8^{\circ },38^{\circ },188^{\circ }$ and $218^{\circ },\frac{I}{I_0}\sim 0.9$
and for $\theta =203^{\circ },\frac{I}{I}\sim \mathrm{0.9.}$
Therefore, angle between direction of polarisation and $x$ axis is $203^{\circ }.$