Question

The distance between a point source of light and a screen is doubled. The intensity will be

• A. four times the original value
• B. two times the original value
• C. half the original value
• D. one quarter of the original value

Answer 1

The correct option is D one quarter of the original value

Intensity, $I\propto \frac{1}{R^2}$, where $R$ is the distance between a point source of light and a screen. If $R$ is doubled then we have
$I^{\prime }\propto \frac{1}{4R^2}=\frac{I}{4}$
Hence intensity will be one quarter of the original value.