When a black body is heated, it emits thermal radiations of different wavelengths or frequency. To explain these radiations, Max Planck put forward a theory known as Planck’s quantum theory. The main points of quantum theory are

**i)** *Substances radiate or absorb energy discontinuously in the form of small packets or bundles of energy.*

**ii)** *The smallest packet of energy is called quantum. In case of light the quantum is known as photon.*

**iii)** The energy of a quantum is directly proportional to the frequency of the radiation .

E α υ

or, E= hυ, where υ is the frequency of radiation and h is Planck’s constant having the value 6.626 x 10^–27 erg – sec or 6.626 x 10^–34 J–sec.

**iv)** A body can radiate or absorb energy in whole number multiples of a quantum hυ, 2hυ,3hυ………..nhυ. where n is the positive integer.

**Neils Bohr** used this theory to explain the structure of atom.

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