Question

The separation between a point source of light and a screen is increased by $1.5\%$. The percentage change in the intensity is:

• A. Increase by $30\%$
• B. Decrease by $3\%$
• C. Increases by $0.75\%$
• D. Decrease by $0.75\%$

Answer 1

The correct option is B Decrease by $3\%$

We know

$I\propto \frac{1}{d^2}⟹I\propto d^{-2}$

where $I$ is the intensity, and $d$ is the separation between a source and screen.

Hence percentage change in intensity

$\%\text{change in}I=(-2)\times \%\text{change in d}⟹=(-2)\times (+1.5)=-3\%$

Hence the percentage change in the intensity is decreases by 3%