What Is The Fundamental Nature Of Light In Physics?

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What Is A Photon? 

sunshine breaking through clouds

A photon, the quantum of electromagnetic radiation, is an elementary particle; a bundle of energy. It is a force carrier of an electromagnetic force. Photons are tiny particles of light that are too small to be seen individually. Photons are best explained by Quantum Physics. The modern concept of photon theory was developed by the great Albert Einstein. The name photon was coined by an American physical chemist Gilbert Newton Lewis in 1926. A photon has zero rest mass; is always in motion and moves at the speed of light ( c= 2.998 x 108 m/s). A photon is usually denoted by the symbol Υ (gamma).


Historical Development Of A Photon

The dispute that whether light is a particle or a wave has been around here for centuries. In the late 1600s, Issac Newton developed and proposed his corpuscular hypothesis that light is a collection of particles. On the other hand just because Newton’s corpuscular theory of light was not able to explain some phenomena such as Refraction, Diffraction, and Interference Of Light; Newton’s contemporaries like Rene Descartes, Christian Huygens, Robert Hooke proposed Wave Theory Of Light. The particle theory of light remained dominant over the wave theory of light chiefly due to the influence of Issac Newton until the start of the 19th century when Thomas young presented his Young’s Double-Slit Experiment that solidified the dominance of the wave theory of light over the particle theory of light.

young’s double-slit experiment demonstrating light is a wave phenomenon
Credit: Wikimedia Commons


In the 19th century, the wave model of light was generally accepted due to the collective works of Thomas Young, August Fresnel, James Clerk Maxwell; that light is an electromagnetic wave. In the end, history repeated itself when wave theory (electromagnetic radiation) of light did not account for all the properties of light. It wasn’t until Max Planck’s hypothesis for Black-Body Radiation and Albert Einstein’s explanation of the Photo-Electric Effect that light energy had to be quantized. The concept of light theory or photon theory came again in general use when in 1923, an American physicist Arthur Holly Compton demonstrated the particle nature of X-rays. This helped in the solidification of Albert Einstein’s hypothesis that light itself is quantized. 

Photon Exhibits Wave-Particle Duality

As we discussed in the above paragraph, that light exhibits the properties of both a particle as well as a wave. In order to explain Wave-Particle Duality, let us take an example– a single photon can behave as a particle having a finite position and momentum (not both at the same time)  like a Billiard Ball, on the other hand, a photon can be refracted by the lens and demonstrate interference like pattern just like a wave in the ocean.

Light as both particle and wave
Credit: Techfrag


In simple terms, one can summarize that a photon’s wave and particle (quanta) qualities are the two detectable aspects of a single phenomenon. Such that, when a photon is treated as a particle– it can be used to calculate frequency, amplitude, or wavelength. On the other hand, if a photon is considered as a wave– it is used in wave mechanics.

Physical Properties Of Photons

According to the photon theory of light:

  1. Photons are massless particles, has no electric charge, can also behave like both a particle and a wave simultaneously.
  2. Photons are gauge boson for electromagnetism having all quantum numbers such as baryon number, lepton number, and flavor quantum number mathematically equal to zero.
  3. photons have zero mass and rest energy; in a vacuum, photons move at the speed of light i.e c = 2.9979 x 108 m/s.
  4. A photon can be destroyed or created when radiation is either absorbed or emitted.
  5. A photon does not obey the Pauli exclusion principle.
  6. A photon follows Bose-Einstein Statistics; making photon a boson having spin number = 1
  7. A photon can be emitted in many natural processes such as when a charge is accelerated, it emits Synchrotron Radiation or during Electron-Positron Annihilation.
  8.   The energy and momentum of a photon are related by E = pc; where C = speed of light and P = magnitude of a momentum vector.


Photon And Standard Model Of Particle Physics

According to the Standard Model Of Particle Physics, photons are responsible for the Electromagnetic Force Of Interaction which is one of the Four Fundamental Forces Of Nature (others are Gravitational Force, Strong Nuclear Force, and Weak Nuclear Force).
The Electromagnetic Forces interact via photons having spin number 1. Photons are responsible for the creation of a magnetic field as well as an electric field. This is the reason that light is called an electromagnetic wave.

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I am a mechanical engineer by profession. Just because of my love for fundamental physics, I switched my career, and therefore I did my postgraduate degree in physics. Right now I am a loner (as ever) and a Physics blogger too. My sole future goal is to do a Ph.D. in theoretical physics, especially in the field of cosmology. Because in my view, every aspect of physics comes within the range of cosmology. And I love traveling, especially the Sole one.

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