# Photon

According to Eiensteinâ€™s quantum theory, light propagates in the bundles (packets or quanta) of energy, each bundle being called a photon and possessing energy.

Energy of photon Energy of photon is given by where c = speed of light, h = Planckâ€™s constant = 6.6 Ã— 10â€“34 J-s, v = frequency in Hz, Î» = wavelength of light.

In electron volt,

Mass of photon Actually rest mass of the photon is zero. But its effective mass is given as
. This mass is also known as kinetic mass of the photon.

Momentum of the photon

Momentum,

Number of emitted photons The number of photons emitted per second from a source of monochromatic radiation of wavelength Î» and power P is given as , where E = energy of each photon.

Intensity of light (I) Energy crossing per unit area normally per second is called intensity or energy flux

i.e.,

At a distance r from a point source of power P, intensity is given by

Number of photons falling per second (n) If P is the power of radiation and E is the energy of a photon then n = P/E.