Question

In a grease spot photometer, light from a lamp with dirty chimney is exactly balanced by a point source distant $10$ 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 the dirty chimney is nearly

• A. $56$ %
• B. $44$ %
• C. $36$ %
• D. $64$ %

Answer 1

The correct option is C $36$ %

More the absorption of light less will be its intensity $I$. But in grease spotmeter $I\propto \frac{1}{{distance}^2}\propto \frac{1}{d^2}$

As, $d_i=10cm,d_2=10-2=8cm$

Percentage of light absorbed $=\frac{d_i^2-d_f^2}{d_i^2}\times 100$$=\frac{100-64}{100}\times 100$$=36$%

So, the answer is option (C).