Optics Question 133

Question: The separation between the screen and a plane mirror is 2r. An isotropic point source of light is placed exactly midway between the mirror and the screen. Assume that mirror reflects 100% of incident light. Then the ratio of illuminances on the screen with and without the mirror is

Options:

A) 10 : 1

B) 2 : 1

C) 10 : 9

D) 9 : 1

Show Answer

Answer:

Correct Answer: C

Solution:

The illuminance on the screen without mirror is $ I _{1}=\frac{L}{r^{2}} $

The illuminance on the screen with mirror is $ I _{2}=\frac{L}{r^{2}}+\frac{L}{{{(3r)}^{2}}}=\frac{10}{9}\times \frac{L}{r^{2}} $

$ \therefore \frac{I _{2}}{I _{1}}=\frac{10}{9}=10:9 $



NCERT Chapter Video Solution

Dual Pane