PHOTOELECTRIC EFFECT
Emission of electrons from a metal surface when exposed to light radiations of appropriate wavelength is called photoelectric effect. The emitted electrons are called photoelectrons.
Work function or threshold energy may be defined as the minimum amount of energy required to eject electrons from a metal surface.
According to Einstein,
Maximum kinetic energy of the ejected electron = absorbed energy – work function
where, 
and 
are threshold frequency and threshold wavelength respectively.
Stopping potential : The minimum potential at which the photoelectric current becomes zero is called stopping potential.
If 
is the stopping potential , then
Some facts of Photoelectric Effect
(i) There is no time lag between incidence of light and emission of photoelectrons.
(ii) For emission of photoelectrons, the frequency of incident light must be equal to or greater than the threshold frequency.
(iii) Rate of emission of photoelectrons from a metal surface is directly proportional to the intensity of incident light.
(iv) The maximum kinetic energy of photoelectrons depends on the frequency of incident radiation; but, it is independent of the intensity of light used.
Illustration: The minimum energy required to overcome the attractive forces electron and surface of Ag metal is 
J. Calculate the maximum K.E. of electron ejected out from Ag which is being exposed to U.V. light of 
= 360 Å
Illustration: A metal was irriadated by light of frequency 
. The photoelectron produced had its KE, 2 times the KE of the photoelectron which was produced when the same metal was irriadated with a light of frequency 
. What is work function.
Sol.
