Q. A lamp is hanging at a height of $4m$ above a table. The lamp is lowered by $1m$. The percentage increase in illuminance will be
A) 40%
B) 64%
C) 78%
D) 92%
Explanation
(c) $I_{1} = \frac{L}{r_{1}^{2}} = \frac{L}{16}$ and $I_{2} = \frac{L}{r_{2}^{2}} = \frac{L}{9}$
% increase in illuminance
$= \frac{I_{2} - I_{1}}{I_{1}} \times 100 = \left( \frac{16}{9} - 1 \right) \times 100 \approx 78\%$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.