Q. A lamp is hanging at a height of 40 cm from the centre of the table. If its height is increased y 10 cm, the illuminance of the lamp will decreased by
A) 10%
B) 20%
C) 27%
D) 36%
Explanation
(d) $\frac{I'}{I} = \frac{40 \times 40}{50 \times 50} = \frac{16}{25}$
$1 - \frac{I'}{I} = 1 - \frac{16}{25} = \frac{9}{25}$
or$\ \ \frac{I - I^{'}}{I} \times 100 = \frac{9}{25} \times 100 = 36\%$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.