Question

A phototube is illuminated by a small bright source placed 100 cm away.
When the same source of light is 50 cm away, what will be the effect on the
number of electrons emitted by photo cathode?

Answer 1

Answer:

number of photo electrons will increase because intensity increase (you can understand the situation as light coming from a far off source will have less intensity then the light coming from near Source)

Answer 2

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{100}{50}})^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 :-

Hope u liked my solution

Pls mark me as brainliest

Thanks.