Q15 of 26 Page 1

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.


(i) If the intensity of the incident light is increased, the photoelectric current will also increase linearly.


i ∝ I


This is because photoelectric current is directly proportional to the number of photoelectrons emitted per second and this is directly proportional to the intensity of the incident radiation.



ii) There should be minimum frequency of light which will strike on the metal surface, for the electron to eject from it. That minimum frequency is called threshold frequency. With the increase of frequency of incident radiation, the kinetic energy of the ejected electron will also increase. But it will not chance the number of electrons ejected. Hence, the anode current will remain same even if the frequency is increased.


(iii) With an increase in the accelerating potential, the photoelectric current will increase first and reach to maximum value when all the electrons get collected at the positive potential plate and then remains constant. The maximum value of the anode current is called the saturation current.



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