Optics Question 134

Question: The separation between the screen and a concave mirror is 2r. An isotropic point source of light is placed exactly midway between the mirror and the point source. Mirror has a radius of curvature r and reflects 100% of the 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: B

Solution:

Illuminance on the screen without mirror is $ I _{1}=\frac{L}{r^{2}} $ Illuminance on the screen with mirror $ I _{2}=\frac{L}{r^{2}}+\frac{L}{r^{2}}=\frac{2L}{r^{2}} $

Therefore $ \frac{I _{2}}{I _{1}}=2:1 $



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