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 $
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