Question

A small plane perfectly reflecting mirror of area $A$ and mass $M$ is hanging vertically with the help of a massless string of length $l$. Light of intensity $I$ is incident horizontally on it. Find the angle made by the string with the vertical.

• A. $\frac{IA}{Mg}$
• B. $\frac{2IA}{cg}$
• C. $\frac{2IA}{cMg}$
• D. $\frac{IA}{cMg}$

Answer 1

The correct option is C $\frac{2IA}{cMg}$



The force exerted by the light on the mirror(perfectly reflecting) is,

$F=\frac{2IA}{c}$

If $T$ is the tension in the string, then for the equilibrium of the mirror,

$T\sin \theta =F$

$T\cos \theta =Mg$

$\therefore \tan \theta =\frac{F}{Mg}$

$\Rightarrow \tan \theta =\frac{2IA}{cMg}$

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(C\right)$ is the correct answer.