Planck’s theory of radiation


What is quantum theory? What did Planck and Einstein contribute to the concept of quantum theory?

Answer:

Planck’s theory of radiation:

The quantum theory based on using the concept of the quantum unit to describe the dynamic properties of subatomic particles and the interactions of matter and radiation. The German physicist Max Planck postulated in 1900 that energy can be emitted or absorbed by matter only in small, discrete units called quanta. Max Planck also give an idea about black body radiation, refers to an ideal body or surface that absorbs all radiant energy without any reflection.  A body at a moderately high temperature-a “red heat”-gives off most of its radiation in the low frequency (red and infrared) regions; a body at a higher temperature-“white heat”-gives off comparatively more radiation in higher frequencies (yellow, green, or blue).   Planck’s theory of radiation

The Spectrum of radiation from a black body is found to be as shown in Figure.

Planck's theory of radiation

(where K: Boltzman’s Constant)

Max Planck made a radical hypothesis that the exchange of energy between oscillation in the walls of the black body and radiation occurs in a discrete manner unlike the classical assumption of continuous exchange.   Planck’s theory of radiation

Planck hypothesized that the oscillations can have energies hv , 2hv , 3hv ,…. where ‘h’ is constant known as Planck’s Constant. Therefore radiation can absorb or transfer energy also in such quanta or bundles. Planck derived the formula

 

Planck's radiation law(2)

Equation (2) is known as Planck’s radiation law. It is in excellent agreement with the observed Spectrum.

The introduction of Planck’s hypothesis is the beginning of Quantum Theory which is one of the pillars of Modern Physics.   Planck’s theory of radiation

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