Optics Question 132
Question: 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 chimney, the point source is moved 2 cm to obtain balance again. The percentage of light absorbed by dirty chimney is nearly
Options:
A) 56%
B) 44%
C) 36%
D) 64%
Show Answer
Answer:
Correct Answer: C
Solution:
The actual luminous intensity of the lamp is $ I _{1} $ whereas the intensity is $ I _{1}^{\prime } $ in the dirty state. I position, $ \frac{I _{1}^{\prime }}{I _{2}}={{( \frac{x}{10} )}^{2}} $ II position, $ \frac{I _{1}}{I _{2}}={{( \frac{x}{8} )}^{2}} $
$ \Rightarrow \frac{I _{1}^{\prime }}{I _{1}}=0.64 $
$ \Rightarrow \ I _{1}^{\prime }=0.64I _{1} $ . Thus, % of light absorbed = 36%.
