Optics Question 478
Question: In a movie hall, the distance between the projector and the screen is increased by 1% illumination on the screen is
[CPMT 1990]
Options:
A) Increased by 1%
B) Decreased by 1%
C) Increased by 2%
D) Decreased by 2%
Show Answer
Answer:
Correct Answer: D
Solution:
$ I=\frac{L}{r^{2}} $
$ \Rightarrow \ \ \frac{dI}{I}\ =-\frac{2dr}{r} $ ( $ \because $ L = constant)
Therefore $ \frac{dI}{I}\times 100\ =-\frac{2\times dr}{r}\times 100 $
$ =-2\times 1=-2% $
