Optics Question 503
Question: A lamp is hanging at a height 40 cm from the centre of a table. If its height is increased by 10 cm the illuminance on the table will decrease by
Options:
A) 10 %
B) 20%
C) 27%
D) 36%
Show Answer
Answer:
Correct Answer: D
Solution:
$ I _{1}=\frac{L}{r _{1}^{2}}=\frac{L}{1600} $ and $ I _{2}=\frac{L}{2500} $
% decease in illuminance $ =\frac{I _{1}-I _{2}}{I _{1}}\times 100 $
$ =( 1-\frac{1600}{2500} )\times 100 $
$ =\frac{900}{2500}\times 100=36 $
