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Friday, 10 March 2017

Dual Nature of Radiation and Matter - II


Photoelectric Effect:
The phenomenon of emission of electrons from the surface of substances (mainly metals), when exposed to electromagnetic radiations of suitable frequency, is called photoelectric effect and the emitted electrons are called photoelectrons.

Maximum K. E of the Photoelectrons Emitted from the Metal Surface:

Cut Off or Stopping Potential:
The value of the retarding potential at which the photoelectric current becomes zero is called cut off or stopping potential for the given frequency of the incident radiation.

Threshold Frequency:
The minimum value of the frequency of incident radiation below which the photoelectric emission stops altogether is called threshold frequency.

Laws of Photoelectric Effect:

a) For a given metal and a radiation of fixed frequency, the number of photoelectrons emitted is proportional to the intensity of incident radiation.

b) For every metal, there is a certain minimum frequency below which no photoelectrons are emitted, howsoever high is the intensity of incident radiation. This frequency is called threshold frequency.

c) For the radiation of frequency higher than the threshold frequency, the maximum kinetic energy of the photoelectrons is directly proportional to the frequency of incident radiation and is independent of the intensity of incident radiation.

d) The photoelectric emission is an instantaneous process.

Einstein’s Theory of Photoelectric Effect:
a) Einstein explained photoelectric effect with the help of Planck’s quantum theory.

b) When a radiation of frequency n  is incident on a metal surface, it is absorbed in the form of discrete packets of energy called quanta or photons.

c) A part of energy hn  of the photon is used in removing the electrons from the metal surface and remaining energy is used in giving kinetic energy to the photoelectron.
d) Einstein’s photoelectric equation is,

Where w0 is the work function of the metal.

e) If n0 is the threshold frequency, then w0 = hn0

f) All the experimental observations can be explained on the basis Einstein’s photoelectric equation

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