Towards the end of 17th century, Newton proposed
his corpuscular theory of light. According to this theory, light consist of
minute fast moving elastic particles known as corpuscles. The phenomena of
interference, diffraction, polarization etc. could not be explained on the
basis of corpuscular theory.
To explain these phenomena, Huygen proposed the wave theory
of light, according to which light travels in the form of waves in a
hypothetical medium called ether.
The wave theory was followed in 1864 by Maxwell’s electromagnetic theory. There
are certain other experimentally observed phenomena such as photoelectric
effect, Compton effect, emission and absorption of light etc. which could not
be explained on the basis of above mentioned theories. These phenomena gave
birth to the quantum theory of light.
The quantum theory of radiation was proposed by in 1900 in order to explain the shape of the
black body radiation curves obtained experimentally. He assumed that the atoms
in the walls of a black body behave like simple harmonic oscillators and each has
a characteristic frequency of oscillation. According to quantum theory, an
oscillator does not radiate energy continuously but the energy is emitted in
discrete packets or quanta.
The oscillator emits energy only when it passes from a higher
energy states to a lower energy state and absorbs energy when it goes from a
lower energy state to higher energy state. No emission or absorption of energy
takes place when the oscillator is in a given state. The smallest amount of
energy which can be emitted or absorbed by the oscillator is hv. The quantum is the basic unit of
energy and cannot be subdivided. Einstein named the quantum of energy as
photon. The photon travels in space with the velocity of light.
The quantum theory of radiation has been able to explain a
number of phenomena which could not be explained by classical theory. In 1905,
Einstein applied it to explain the photoelectric effect. In 1913 Bohr used the
quantum concept of radiation in his analysis of atomic structure and the origin
of spectral lines. Sevaral other phenomena e.g. Compton effect, stark effect,
Zeeman effect and the variation of specific heat with temperature, have also
been explained on the basis of quantum theory.
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