# **Plank’s Quantum Theory of Radiations**:

**In 1900, a German Physicist Max Plank had studied radiations emitted by hot black body and proposed quantum theory of radiations. **

**Main Postulates of Plank’s Quantum Theory of Radiations:**

**Postulates of the theory are:**

- Energy radiated by hot bodies is not continuous, rather it is emitted in the form of “energy packets” or “energy pulse” known as quanta (singular is quantum). The particle of energy (radiations) is also called photon.
- Each quantum has a definite amount of energy. This energy of the quantum is directly proportional to frequency of radiations.

**E ∝ ﬠ**

Or

E = h ﬠ

- According to Plank, energy of a quantum or photon is given as:

**E= h ﬠ **

Where E = energy

h = Plank’s constant

= 6.6262 x 10^{-34} J.s

But ﬠ = c / λ

So E = hc / λ

Equation-1 shows that energy is directly proportional to frequency of radiations.

E ∝ ﬠ

While equation-2 shows that energy is inversely proportional to wave length.

E ∝ 1 / λ

- The energy emitted or absorbed by a body is always whole number multiple of quantum i.e. E
_{n}= n h ﬠ where n = 1, 2, 3,……..

This means energy is quantized.

Energy of photon can also be given as:

_

E = hc `ﬠ`

In 1905, Einstein also gave similar idea of particle nature of radiations.

He gave equation :

** E = mc ^{2}**

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