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Planck’s Quantum Theory


The failure of the classical electromagnetic theory of radiation to explain the phenomenon of photoelectric effect led Max Planck (1 901) to propose a new theory known as Quantum theory of radiation. The main points of this theory are:

1. Radiant energy is emitted or absorbed not continuously but discontinuously in the form of small packets of energy called quanta.

Each wave packet or quantum is associated with definite amount of energy. In case of light, the quantum of energy is often called photon.

2. The amount of energy associated with a quantum of radiation is proportional to the frequency of radiation

E  ∞ v

E = hv

where his a constant known as Planck’s constant and is equal to 6.625 x l0-34 joule-sec.

The energy of a photon of light in terms of wavelength A and velocity of light c, is given as

E = hv

E = h. c / λ                               (c = vλ ; v c /λ)

3. A body can emit or absorb energy only in terms of the integral multiples of quantum, i.e.,


where n = 1, 2, 3, 4, …..

This means that a body can emit or absorb energy as hv, 2hv, 3hv, 4hv …. . but it cannot emit or absorb energy in fractional values of hv such as 1.5hv or 2.5hv.