Q. In a photometer, two sources of light when placed at 30 cm and 50 cm respectively produce shadows of equal intensities. Their candle powers are in the ratio of
A) $\frac{9}{25}$
B) $\frac{16}{25}$
C) $\frac{3}{5}$
D) $\frac{5}{3}$
Explanation
(a) When the screen is equally illuminated,
$E_{1} = E_{2}$
Or $\ \frac{I_{1}}{r_{1}^{2}} = \frac{I_{2}}{r_{2}^{2}}\ or\ \frac{I_{1}}{I_{2}} = \frac{r_{1}^{2}}{r_{2}^{2}} = \frac{30 \times 30}{50 \times 50} = \frac{9}{25}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.