Question

A lamp is hanging at a height of $40m$ from the centre of a table. If its height is increased by $10cm$, the illuminance on the table will decrease by

• A. $10$%
• B. $20$%
• C. $27$%
• D. $36$%

Answer 1

The correct option is D $36$%

$\frac{E_2}{E_1}=\frac{r_1^2}{r_2^2}=\frac{{40}^2}{{50}^2}=\frac{16}{25}$

So, $E_2=\frac{16}{25}{E}_1$

So, decrease in intensity $=\frac{E_1-E_2}{E_1}\times 100=36\%$