Q. In a grease spot photometer, light from a lamp with dirty chimney is exactly balanced by a point source distance 10 cm from the grease spot. On clearing the dirty chimney, the point source is moved 2 cm to obtain a balance again. Then the percentage of light absorbed by the dirty chimney is nearly
A) 64%
B) 36%
C) 44%
D) 56%
Explanation
(b) $r_{1} = 10\ cm,r_{2} = 8\ cm$
$\frac{I_{1}}{I_{2}} = \frac{64}{100},1 - \frac{I_{1}}{I_{2}} = 1 - \frac{64}{100}$
Or $\frac{I_{2} - I_{1}}{I_{2}} = \frac{36}{100}$
Or $\frac{I_{2} - I_{1}}{I_{2}} \times 100 = 36\%$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.