Question

Light of intensity ‘I’ and frequency ‘v’ is incident on a photosensitive surface and causes photoelectric emission. What will be the effect on anode current when (i) the intensity of light is gradually increased, (ii) the frequency of incident radiation is increased, and (iii) the anode potential is increased? In each case, all other factors remain the same.
Explain, giving justification in each case.

Answer 1

Photoelectric emission is defined as a process by which electrons leave the surface of photoelectric materials when light of a particular frequency strikes it.
Intensity of light is the measure of its brightness.

Complete step by step solution:
Part (i):
If the anode potential and frequency of the incident radiation is kept constant and the intensity of incident light is increased, then the anode current would show a linear increase in its magnitude.
Since the anode current is directly proportional to the number of electrons emitted per second.

$anode{\text{ }}current{\text{ }}\alpha {\text{ }}number{\text{ }}of{\text{ }}electrons{\text{ }}emitted{\text{ }}per{\text{ }}second$

And the number of electrons emitted per second is directly proportional to the intensity of the incident radiation.

$number{\text{ }}of{\text{ }}electrons{\text{ }}emitted{\text{ }}per{\text{ }}second{\text{ }}\alpha {\text{ }}intensity{\text{ }}of{\text{ }}the{\text{ }}incident{\text{ }}radiation$

So, we can conclude from the above two equations that intensity of incident light is directly proportional to anode current.
Part(ii):
The frequency of incident light must be above the threshold frequency (i.e. minimum cut off frequency of the incident radiation) for photoelectric emissions to occur.
But we know that the threshold frequency is independent of the intensity of the incident light.
Hence, with an increase in the frequency of the incident radiation, the kinetic energy of the ejected electrons increases which is independent of the number of electrons ejected.
So, with the increase in the frequency of incident radiation, there will not be any change in the anode current.
Part(iii):
With the increase in anode potential, the photoelectric current would first increase, then reach its maximum value (when all the electrons are collected at the positive plate) and finally remains constant (at its maximum value).

Note: In order to solve such theoretical kind of questions, one must be familiar with the definition of all the terms mentioned in the question.