Question

A photoelectric cell is illuminated by a point source of light 1 m away. When the
source is shifted to 2m then​

Answer 1

Solution :-

Small bright source of light can be considered as a spherical light source.

We know that :-

1. Intensity of light emitted by a spherical source at a distance 'r' is inversely proportional to r².
2. Also, number of photons incident on the photo tube will vary directly to the intensity of light at a distance 'r'.
3. And, number of emitted electrons will also depend directly on number of emitted photons.

Concluding all three statements, we came to the fact that number of emitted electrons from the influence of a point source at a distance 'r' is inversely proportional to r².

Hence,

$\frac{(Number\ of\ incident\ electrons\ initially)}{(Number\ of\ incident\ electrons\ finally)} =({\frac{Initially\ separation}{Final\ separation}})^2$

$\frac{(Number\ of\ incident\ electrons\ initially)}{(Number\ of\ incident\ electrons\ finally)} =({\frac{1}{(2)}})^2$$Number\ of\ incident\ electrons\ initially\times4=Number\ of\ incident\ electrons\ finally$Hence, the number of electrons emitted by photocathode would decrease by a factor of 4.

Remarks :-

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