Q. The separation between the screen and a plane mirror is $2r$. An isotopic point source of light is placed exactly mid way between the mirror and the screen. Assume that mirror reflects 100% of incident light. Then the ratio of illuminance on the screen with and without the mirror is
A) 10:1
B) 2:1
C) 10:9
D) 9:1
Explanation
(c) $E_{1} = \frac{I}{r^{2}},E_{2} = \frac{I}{r^{2}} + \frac{I}{9r^{2}}$
$\therefore\frac{E_{1}}{E_{2}} = \frac{I}{r^{2}} \times \frac{9r^{2}}{10I}$
Or $\frac{E_{2}}{E_{1}} = \frac{10}{9}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.