Write Einstein’s photoelectric equation and mention which important features in photoelectric effect can be explained with the help of this equation. The maximum kinetic energy of the photoelectrons gets doubled when the wavelength of light incident on the surface changes from 
to 
. Derive the expressions for the threshold wavelength 
and work function for the metal surface.
Einstein’s photoelectric equation is given as: 
where, h
is the energy of the photon,
, is the work function of the metal,
is the maximum kinetic energy of the emitted photoelectron.
Maximum kinetic energy is given by,
;
where,
is the threshold frequency of the metal surface.
Features of the equation of photo-electric effect are:
a) When a photon of suitable radiation falls on the metal surface, a photoelectron is ejected. If the intensity of the light is increased number of photons incident on the metal surface increases and hence, the number of photo-electrons ejected also increases. This implies, photo current is proportional to intensity and radiation.
b) If frequency is less than the threshold frequency, i.e., the, photoelectric emission does not take place for incident radiation.
c) When frequency of the incident light is greater than the threshold frequency, maximum K.E increases. This implies, maximum K.E of photoelectrons depends upon the frequency of incident light.
Einstein’s equation corresponding to wavelength 
Einstein’s equation corresponding to wavelength 
is, 
From equation (1) and (2), we have
where, 
is the threshold wavelength.
