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Particle nature of the radiation (or light)

 Particle nature of radiation (Or light) 


Electromagnetic wave theory explains the properties of light such as interference , diffraction and polarization . But certain phenomena like photoelectric effect, black body radiation, line spectra of atoms with special reference to hydrogen, variation of heat capacity of solids as a function of temperature, could only be explained by quantum (photon) theory of radiation in which radiation is considered as a particle nature.  

It means that radiation has a dual nature, both as a particle and as a wave. By taking an example it could be easier to explain. 
Example: when we see an object, both the aspects are important. The gathering and focusing of light by the eye-lens is well described by the wave picture, while absorption of light by the retina is described by the photon picture of light. 

In 1901,Planck put forward the particle nature of electromagnetic radiations(Planck's Quantum Theory)  to explain the phenomenon of black body radiation and photoelectric effect.

According to this theory, energy is not emitted nor absorbed continuously. Energy is lost or gained in the form of small packets (bundles) called photon. Each such packet of energy is called a quantum. In case of light, the quantum of energy is called a photon. 

Following is the relation between energy of radiation (E), it's wavelength (λ) and frequency(υ).

E=hυ=hc/λ
or
υ=c/λ

E is the energy of one photon and is proportion to the frequency of radiation, h is called Planck's constant. It is a universal constant, c is the velocity of light radiation. 

Thus, energy emitted or absorbed may be hυ  , 2hυ ,  3hυ  ...... Energy less than a quantum (hυ) is neither lost nor gained. hυ is called one quantum. 

A definite value of energy as well as momentum is a strong evidence for the particle nature of radiation. 


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