Using Einstein's photoelectric equation show how the cut -off voltage and threshold frequency for a given photosensitive material can be determined with the help of a suitable plot/graph.

Explanation

When light of sufficiently small wavelength is incident on a metal surface, electrons are ejected from the metal. This phenomenon is called the photoelectric effcect.

(i) The cathode is illminated with light of some fixed frequency `v` and fixed Intensity `I_(1)`. A small photoelectric current is observed due to few electron that reach anode just because they have sufficiently large velocity of emision. If we made the potential of the anode negative with respect to cathode then the eelctrons emitted by cathode then the electorns emiited by cathode are repelled. Some electrons even go back to the cathode so that the current decreases. At a certain value of this negative potential, the current is completely stopped. Then least value of this anode potential which just stops the photocurrent is called cut off potential or stopping potential.
(ii) For a given material, there is a certain minimum frequency that if the incident has a frequency below this threshold, no photoeelctirc emiision will take place, how so ever intense the radiation may be falling.
(iii) According to Einstein's photoelectric equation, maximum `K.E.` is given as
`K.E_("max") = (h c)/(lambda)-phi`
Where `lambda` is wavelength and `v` is corresponding frequency and `phi` is work function.
We expose a material to lights of various frequencies and thus photoelectric current is observed and cut off potential needed to reduce this curretn to Zero is noted. A graph is plotted and that is stright line. ltbgt At `v_(0)` the stopping potential is zero this means this frequency the incident light not able to eject electrons this is threshold frequency. Extended this line gives the cut off potential to make the phob current zero which is `(-phi)/(e )`