Planck’s Quantum Theory

By the end of 19th century, scientists were able to explain most of the natural phenomenon by Newton’s Laws of classical mechanics or classical theory. By this time matter and energy used to considered as distinct and unrelated. Scientists were able to define the properties of radiant energy by Maxwell’s equations given by James Clerk Maxwell, a Scottish Physics in 1873. However, by the 20th century scientists were able to discover that phenomenon such as black body radiation, photoelectric effect can’t be explained by classical theory or classical mechanics. During this time, German physicist Max Planck put forward his theory of quantized nature of energy of electromagnetic wave. In this article we will discuss Max Planck’s quantum theory of radiation or quantum theory of radiation, black body radiation, electromagnetic radiation, evidences for a particle theory of energy. 

Electromagnetic Radiation 

It is a form of energy that can propagate in vacuum or material medium and shows both wave like and particle like properties. Radio waves, microwaves, infrared, visible light, UV-rays, X-rays, gamma rays are electromagnetic radiation. 

Black Body Radiation 

According the temperature, all objects emit electromagnetic radiations. Objects having low temperature emit radio or microwaves (low frequency waves) while objects having high temperature emit visible or ultraviolet light or even higher frequency radiations. 

A black body is an idealized object that can absorb all electromagnetic radiation which comes in contact of it. After this it starts emitting thermal radiation in a continuous spectrum according to its temperature. The radiation which a black body emits is called black body radiation. Stars almost behave like black body. 

Intensity of the radiation vary according to the wavelength and temperature of the object. At a given temperature, intensity of a light varies according to the wavelength. This phenomenon was not explained by classical theory or Maxwell’s equation. Hence, Max Planck put forward his theory of quantization of energy or Planck’s quantum theory of radiation to explain this phenomenon. 

What is Planck’s Quantum Theory? 

Planck’s quantum theory explains emission and absorption of radiation. Postulates of Planck’s quantum theory are as follows – 

  1. Matter radiate energy or absorb energy in discrete quantities discontinuously in the form of small packets or bundles. 

  2. The smallest bundle or packet of energy is known as quantum. In case of light, a quantum of light is known as a photon. 

  3. The energy of the quantum absorbed or emitted is directly proportional to the frequency of the radiation. So, energy of the radiation is expressed in terms of frequency as follows- 

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  1. A body or matter can radiate energy or absorb energy in whole number multiples of a quantum as nhv. Where n is a positive integer. So, energy can be absorbed or radiated as hv, 2hv, 3hv, 4hv……etc. not in the form of 1.5hv, 2.5hv…etc. 

After Max Planck, German physicist Albert Einstein revisited the theory and explained photoelectric effect. 

Evidences in Support of Planck’s Quantum Theory 

Many experiments were performed to analyze Planck’s quantum theory. All experimental observations supported and worked as strong evidence for quantum theory. All shows that energy of electron motion in matter is quantized. A prism can separate the light according to their wavelengths. If light behave only as a wave, then prism should give a continuous rainbow. This also supports Planck’s Quantum Theory. Emission spectrum of nitrogen gas also supports Planck’s quantum theory of radiation.  

Applications of Planck’s Quantum Theory 

Planck’s quantum theory is the fundamental theory of quantum mechanics. So, it has applications in all those fields where quantum mechanics is being used. It has applications in electrical appliances, medical field, quantum computing, lasers, quantum cryptography etc.