<h2><center>Nature of Light </center></h2>

Light gives a sensation of vision to us. We see things around us due to light. It is a form of energy but what exactly is nature of light? Different theories are put forward by different scientists about nature of light. I am going to write about these theories about nature of light. 

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# Newton’s Corpuscular Theory

The Corpuscular theory was one of earliest attempts to describe the nature of light. This theory was put forward by Sir Isaac Newton in 1678 AD. According to this theory, light consists of small particles called ‘Corpuscles’. These small particles are produced by luminous sources. They travel in all direction with high velocity. When these particles enter into the retina of an eye, these give the sensation of vision. Different colors are due to different sizes of corpuscles.  This theory tried to explain the phenomena of refraction of light. According to this theory, ‘When light travels in a denser medium, corpuscles are attracted by the particles of a denser medium due to which direction of light changes and velocity increases’. But, the fact is the velocity of light decreases in a denser medium. Also, this theory fails to explain phenomena like interference, diffraction etc. So, this theory was discarded as an inaccurate theory.

# Wave Theory

In 1690, Dutch physicist Christian Huygens proposed wave theory of light. According to this theory, light is from of energy and travels as a wave. Each point of light acts as a source and sends wave in all direction. It travels in a hypothetical medium called Aether (or ether). In 1801, Thomas Young demonstrated the interference of two light beams which is strong evidence for the wave theory of light. This theory successfully explained reflection, refraction interference and diffraction. Huygens considered light as a longitudinal wave travelling in a hypothetical medium called Aether. Later, Young and Fresnel assumed light as a wave. The hypothetical medium Aether was a matter of interest to many scientists like Maxwell, Tesla, Huygens etc. The existence of Aether was disproved by Michelson Morley experiment.

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# Electromagnetic Theory

In 1860 AD, from basic four Maxwell equations, Maxwell found a mathematical expression for the speed of electromagnetic waves as http://quicklatex.com/cache3/ef/ql_5c9b2890b78cbe66118a541daeff36ef_l3.png.

where http://quicklatex.com/cache3/3d/ql_b2a4b734c33e3c59d62d61e4fc38703d_l3.png is the permeability and 
http://quicklatex.com/cache3/13/ql_1a600e59e1f1390e8497086258e21413_l3.png is the permittivity of the space.

In a vacuum, this value was calculated to be http://quicklatex.com/cache3/a6/ql_064d1057d0d7dad346b0b5edf143dea6_l3.png m/s. This is exactly the speed of light observed from the experiment. Maxwell suggested that this isn’t coincidence rather light is an electromagnetic wave. An electromagnetic wave consists of changing electric and magnetic fields which are perpendicular to each other. So, the light wave is transverse in nature.

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# Quantum Theory of Radiation

Wave theory of light couldn’t explain certain phenomena such as photoelectric effect, atomic excitation, Compton Effect etc. In 1905 AD, Albert Einstein proposed a new theory of light called quantum theory based on the assumptions of Max Planck. According to this theory, light consists of a tiny packet of energy called quanta or photons. The energy of each ‘Quanta’ given by;

E = h f

E = Energy of each quanta 
h = Planck's Constant
f = frequency of radiation

The absorption and the release of energy is always in the integral multiple of this energy.

# Dual Nature of Light

Some experiments show wave nature of light whereas the some show particle nature of light. Wave theory light successfully explained the interference, diffraction, polarization etc but it couldn't explain the photoelectric effect, Compton scattering, atomic excitation etc. Photoelectric effect, Compton scattering, Atomic excitation can be explained by the quantum theory of radiation but interference, diffraction, polarization can’t be explained. Instead of considering light as wave or particle we must treat light as having both particle and wave nature. This is called Dual nature of light. 

# References

1. Advanced Level Physics; Resnick and Halliday

2. Univeristy Physics; Hugh young

3. Concepts of Physics; HC Verma 

maths coded on: quicklatex.com