Define the terms threshold frequency and stopping potential in relation to the phenomenon of photoelectric effect. How is the photoelectric current affected on increasing the (i) frequency (ii) intensity of the incident radiations and why?

 

Dear Student,

Photoelectric effect is the phenomenon of emission of electrons mainly from metal surfaces exposed to light energy of suitable frequency.

Threshold frequency(${\nu }_0$) is that minimum frequency of  electromagnetic radiation below which photo electric emission does not takes place.


Consider the figure in which photo electrons are emitted after a suitable frequency of electromagnetic (em ) radiation are incident upon electrode C and reach A. A certain potential difference is applied across the electrodes such that A is positive and the potential on A is increased such that all the photoelectrons emitted by C reach A and the photoelectric current (recorded by ammeter) reaches a maximum value(saturation current). After this no increase in A will result in an increase in current.

Now the potential on A is decreased in such a way that A is negative with respect to electrode C. This retards the electrons with lower kinetic energy (K.E) to reach A but electrons with higher K.E still reach A. As the potential is further decreased across A till it reaches a voltage V₀ at which no photo electrons reach A. At V₀ photoelectric effect is zero. The minimum negative potential that is applied to electrode A for which there is no photoelectric current is known as the Stopping (cutoff) potential. It is the direct measure of K.E of the photo electrons such that

K.E_max = eV_​0          

In Photoelectric  it is observed if the intensity of the incident radiation is increased, keeping the frequency constant, the photo electric current is also increased.

Photoelectric current does not change with the change in frequency of the incident radiation provided the a frequency at or above threshold frequency is maintained.






Regards