Question

(a) Describe briefly three experimentally observed features in the phenomenon of photoelectric effect.
(b) Discuss briefly how wave theory of light cannot explain these features
OR
(a) Write the important properties of photons which are used to establish Einstein's photoelectric equation.
(b) Use this equation to explain the concept of (i) threshold frequency of (ii) stopping potential

Answer 1

$a)$ The three experimentally observed features in the phenomenon of photoelectric effect are -

i) Threshold frequency: The photoelectric effect will occur when the incident frequency is greater or equal to the threshold frequency for a given metal i.e $\nu \ge {\nu }_0$.

ii) The maximum kinetic energy of photoelectron: When the incident frequency is greater than the threshold frequency, the maximum kinetic energy is proportional to $(\nu -{\nu }_0)$.

iii) Effect of intensity of light : The number of photon incident per unit time per unit area increases with the increase of intensity of incident light. Thus, more the number of photoelectrons increases the photocurrent till it will be saturated.

$b)$ We can not explain these by using wave theory of light because there are following reasons-

i) The instantaneous ejection of photoelectrons.

ii) The existence of threshold frequency for a metal surface.

iii) The kinetic energy of photoelectrons is independent of the intensity of light and depends on its frequency.