Photon (pl. photons) is defined as a quantum (pl. quanta) of energy. This concept led to the establishment of the particle nature of light.
Introduction to Photons
In the early days of Physics before Einstein, scientists considered light used to behave as a wave. They engaged in discussions about whether light was a transverse or a longitudinal wave. In the 19th century, theoretical physicist Einstein described light as a particle concerning his theory of the Photoelectric effect. He referenced Planck's explanation of black body radiation $\rm \left ( \rm E = h \nu \right )$ to explain the particle nature of light.
Light contains several quanta of energy called photons. The energy of each photon is given by Planck's law, $\rm \left ( \rm E = h \nu \right )$, where $\rm h$ is Planck's constant and $\rm \nu$ is the frequency of radiation. The momentum of each photon is $\rm P = \frac{E}{c} = \frac{h \nu}{c} = \frac{h}{\lambda}$, where $\rm c$ denotes the speed of light in vacuum and $\rm \lambda$ represents the wavelength of photon.
Characteristics of Photons
- Photon travels at the speed of light.
- It has no rest mass.
- A photon has no charge; it is chargeless. Therefore, it is not deflected by electric of magnetic fields.
- Photons can cause a photoelectric effect.
- Energy of a photon is wholly kinetic.
With the particle nature of light, phenomena like the Photoelectric effect and Compton scattering have been explained successfully, which were otherwise not explained by the wave theory of light.