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 $
