question_answer

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 

A)            56%                                          

B)            44%

C)            36%                                          

D)            64%

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, \[\frac{{{I}_{1}}^{\prime }}{{{I}_{2}}}={{\left( \frac{x}{10} \right)}^{2}}\] II position, \[\frac{{{I}_{1}}}{{{I}_{2}}}={{\left( \frac{x}{8} \right)}^{2}}\]  \[\Rightarrow \frac{{{I}_{1}}^{\prime }}{{{I}_{1}}}=0.64\] \[\Rightarrow \ {{I}_{1}}^{\prime }=0.64\,{{I}_{1}}\]. Thus, % of light absorbed = 36%.