The Story of Photon

Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter


In this blog we will speak in detail about interesting behaviour of light. Light has the ability to show wave nature as well as particle nature but how? Let’s start knowing.
The phenomenon of photoelectric effect confirmed that light is made of small energy packets called quanta. Each quanta have energy h 𝜈. Einstein also suggested that light quantum can also have momentum h 𝜈 /c. With definite value of energy as well as momentum Einstein arrived at the conclusion that light quantum can be associated with a particle. This particle was named as photon.

Compton effect photon

Compton Effect


In 1924, A.H.Compton performed an experiment. He allowed to strike X rays over electron present in carbon target. X rays were observed to be scattered. However, scattered wavelength is longer than wavelength before strike. Shift of wavelength is well explained with particle nature rather than the classical wave theory. Shift of wavelength of X – ray, when it interacts with matter is called Compton Effect. Thus, it is strong evidence that light has particle nature.



Radiations when interact with matter, behaves as if particles called as photon.
Each photon have energy E = h 𝜈. The photon travel with speed of light c and momentum p =h 𝜈/c. Moreover, all photons of light have same energy and momentum irrespective of intensity of radiation. Increase in intensity just results in increase in the number of photons per second crossing a given area with which each photon have same energy. Hence, photon energy is independent of intensity of radiation.

Photons are electrically neutral and don’t deflect by electric and magnetic fields. In collision of photon particle, total energy and total momentum remains conserved. However, during collision number of photons may not be conserved. As during collision photon may be absorb or new photon may be created.

In 1921, Albert Einstein was awarded the Nobel Prize in Physics for his contribution to theoretical physics and the photoelectric effect. In 1923, Millikan was awarded the Nobel Prize in physics for his work on the elementary charge of electricity and on the photoelectric effect.

Keywords: Photoelectric effect, Momentum, Compton Effect, Quanta, X ray

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