Quantum Mechanics and Light



Light: Light is electromagnetic radiation of a wavelength that is visible to the human eye (in a range from about 380 or 400 nanometers to about 760 or 780 nm).  In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.
To describe light there are 4 theories:

  1. 1.     Corpuscular theory of light
  2. 2.     Wave theory
  3. 3.     Electromagnetic theory
  4. 4.     Quantum theory of light

 

Corpuscular theory of light: In 1675 Isaac Newton explained that light was composed of corpuscles   (particles of matter) which were emitted in all directions from a source.
It can explain Reflection, Refraction, Photoelectric effect of light. But can not explain Interference, Diffraction, Polarization effect of light.

 

Wave theory of light: Christian Huygens worked out wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.
It can explain Reflection (প্রতিফলন), Refraction (প্রতিসরন), Interference (ব্যতিচার), Diffraction (অপর্বতন) and Polarization (সমবর্তন). But can not explain the Photoelectric effect of light.
Electromagnetic theory:


 

 

 

Quantum theory of light:  In 1900, Max Planck explained Quantum theory of light after plank’s Albert Einstein expend it. Planck's theory was based on the idea that black bodies emit light only as discrete bundles or packets of energy. These packets were called quanta, and the particle of light was given the name photon. A photon has an energy, E, proportional to its frequency, f, by

    

Where h is Planck's constant and that is, λ is the wavelength and c is the speed of light. 
Summary:  Quantum theory of light says that light exists as tiny packets, or particles, which are called photons. 



It can explain Photoelectric effect of light. But this theory did not explain Interference, Diffraction etc. of light. 


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