QUANTUM THEORY OF RADIATION:

The wave theory of transmission of radiant energy appeared to imply that energy was emitted

(or absorbed) in continuous waves. In 1900 Max Planck studied the spectral lines obtained

from hot-body radiations at different temperatures. According to him, light radiation was produced

discontinuously by the molecules of the hot body, each of which was vibrating with a specific frequency

which increased with temperature. Thus Planck proposed a new theory that a hot body radiates

energy not in continuous waves but in small units of waves. The ‘unit wave’ or ‘pulse of energy’ is

called Quantum (plural, quanta). In 1905 Albert Einstein showed that light radiations emitted by

‘excited’ atoms or molecules were also transmitted as particles or quanta of energy. These light

quanta are called photons.

The general Quantum Theory of Electromagnetic Radiation in its present form may be stated

as :

(1) When atoms or molecules absorb or emit radiant energy, they do so in separate ‘units

of waves’ called quanta or photons. Thus light radiations obtained from energised or

‘excited atoms’ consist of a stream of photons and not continuous waves.

(2) The energy, E, of a quantum or photon is given by the relation

E = hν ...(1)

where ν is the frequency of the emitted radiation, and h the Planck’s Constant. The value

of h = 6.62 × 10– 27 erg sec. or 6.62 × 10– 34 J sec.

We know that c, the velocity of radiation, is given by the equation

c = λν ...(2)

Substituting the value of ν from (2) in (1), we can write

E = hc/λ

Thus the magnitude of a quantum or photon of energy is directly proportional to the

frequency of the radiant energy, or is inversely proportional to its wavelength, λλλ.

(3) An atom or molecule can emit (or absorb) either one quantum of energy (hννν) or any

whole number multiple of this unit.

Thus radiant energy can be emitted as hν, 2hν, 3hν, and so on, but never as 1.5 hν, 3.27 hν,

5.9 hν, or any other fractional value of hν i.e. nhν

Quantum theory provided admirably a basis for explaining the photoelectric effect, atomic

spectra and also helped in understanding the modern concepts of atomic and molecular structure

Tuesday, 4 May 2021