Before study the** properties of photon** we must have to know some basic introduction of photon. In 1905, Einstein explained photoelectric effect by considering light consisted of quantum of energy **E = hv**, Where **v** is the frequency of the radiation and **h=6.63*10 ^{-34} Js** is Planck’s constant.

Gilbert Lewis (an American chemist) in 1926 coined the word “photon” to describe the quantum of energy.

## What is Photon

Photon is a **packet of energy** or quantum of energy emitted by a source, when the electron of an atom of the source goes from higher energy state to lower energy state as shown in 1^{st} figure below and absorbed by a source, when the electron of the atom of the source goes from lower energy state to higher energy state as shown in 2^{nd } figure.

Now we will study some important **properties of Photon**.

**Properties of Photon**

- Photons are indivisible packets of electromagnetic energy.
- All photons travel with a speed of light in vacuum (i.e. c = 3*10
^{8}m s^{-1}). - Photons retain their identity or exist until completely absorbed by atoms of a substance.
- The rest mass of a photon is zero i.e. m
_{0}=0. - The total energy (E) of a particle is given by

Where, m_{0} is rest mass of the particle, p is its momentum and c= speed of light in vacuum.

For s photon, m_{0}=0. Therefore,

Which is the momentum of a photon.

- The effective mass or kinetic mass of a photon is given by

- The energy content or size of a photon is directly proportional to the frequency of the radiation. That is,

Thus, the size of photons of different radiation is different. For example, energy content or size of photon of red light is smaller than the photon of violet light because wavelength (λ) of red light is greater than the wavelength of violet light.

- The intensity of radiation or light (I) is equal to the number of photons (N) crossing unit area per unit time multiplied by the energy (E = hv) of a photon. That is,

**I = NE = Nhv**

Thus, for a given frequency of radiation , intensity of radiation is directly proportional to the number of photons.

- Photons travel in a straight line.
- Photons are electrically neutral and hence they are not deflected by electric and magnetic fields.
- Each photon has an intrinsic angular momentum or spin equal to h.
- Photons are emitted or produced in the process of transition of atoms, molecules, ions or atomic nuclei from the higher or excited energy state to the lower or less energy state.
- Photons are also emitted, when a charged particle is accelerated or decelerated.