Planck’s Quantum Theory | Discovery, Advantages

Definition Planck’s Quantum Theory

Planck’s Quantum Theory: All electromagnetic radiations are forms of energy. The electromagnetic wave theory believed in continuous generation of radiant energy, i.e., the energy may be emitted or absorbed in any value from infinitely small to infinitely large.

Planck's Quantum Theory

However, this theory could not explain the experimental results of many phenomena, such as black body radiation and the photoelectric effect. At the beginning of the twentieth century Max Planck in 1901, gave a new revolutionary theory known as the quantum theory of radiation.

Discovery of Planck’s Quantum Theory

Discovery of Planck's Quantum Theory

However, by the 20th century, scientists discovered that phenomenon such as black body radiation, the photoelectric effect can’t be explained by classical theory or classical mechanics.

During this time, the German physicist Max Planck put forward his theory of the quantized nature of the energy of the electromagnetic wave.

The main Features of Planck’s Quantum Theory of Radiation

main Features of Planck's Quantum Theory
  1. Radiant energy is not emitted or absorbed continuously but discontinuously in the form of small packets of energy called quanta. Each such quantum is associated with a definite amount of energy. In the case of light, the quantum of energy is often called a photon.

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

                                   Ε ∝ ν or E = hν …(i)

where the proportionality is constant, h is a universal constant known as Planck’s constant. It has a value of 6.626 × 10–34 J s or 3.99 × 10–13 kJ sec mol–1. This relation was found to be valid for all types of electromagnetic radiation.

3. The total amount of energy emitted or absorbed by a body will be some whole number multiple of quantum, i.e.,

                              E = nhν or E = nhc/λ … (ii)

where n is an integer such as 1, 2, 3, ….

This means that a body can emit or absorb energy equal to hν, 2 hν, 3 hν…. or any other integral multiple of hν but cannot emit or absorb energy equal to 1.6 hν, 3.2 hν or any other fractional value of hν. 

The relation [equations (i) and (ii)] give the relation between the energy of the radiation and its wavelength or frequency.

It shows that the higher the frequency (or, the lower the wavelength), the more energetic are the corresponding photons.

For example, a photon of violet light will be of more energy than that of red light because the former is of larger frequency. However the concept of energy packets of light supports the corpuscular character.

It also explained the intensity distribution in radiation from a black body as a function of frequency at different temperatures.

Advantages of Planck’s Theory

Advantages of Planck's Theory

1. It explains the energy spectrum of black body radiation.

2. It is also used to deduce Wien’s displacement law and Rayleigh Jean’s Law.

3. It also introduces a new concept, i.e., energy is absorbed or emitted discretely in terms of quanta of the magnitude of hv i.e., E= nhv.

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