Optics Question 513
Question: A lamp is hanging at a height of 4m above a table. The lamp is lowered by 1m. The percentage increase in illuminace will be
Options:
A) 40 %
B) 64%
C) 78%
D) 92%
Show Answer
Answer:
Correct Answer: C
Solution:
$ I _{1}=\frac{L}{r _{1}^{2}}=\frac{L}{16} $
and $ I _{2}=\frac{L}{r _{2}^{2}}=\frac{L}{9} $
% increase in illuminance $ =\frac{I _{2}-I _{1}}{I}\times 100 $
$ =( \frac{16}{9}-1 )\times 100 $
$ \approx 78% $
