A photo cathode is illuminated by a very small bright source placed 1 m away. When the same source of light is placed 0.5 m away, the number of electrons emitted by the photo cathode would become

four times

The correct answer is Option (1) → four times

Intensity ∝ 1/r²

When distance changes from 1 m to 0.5 m:

$\frac{ New intensity }{ Old intensity } = \frac{(1 / 0.5²)}{(1 / 1²)} = 4$

∴ Number of electrons emitted ∝ Intensity ⇒ Increases 4 times

Answer: 4 times