(Based on my Quora Answer)
A photon is a particle and it doesn’t have mass. It does however have momentum (p).
Source: Hyperphysics
This is shown in the diagram above of Compton Scattering. The collision of the photon with an electron at rest increases the wavelength of the scattered photon and provides the electron with momentum (shown with relativistic correction). Photons have energy that is proportional to its frequency. This energy of a photon can be calculated by multiplying the frequency (v…Greek symbol nu) by Planck’s constant (h).
The momentum of a photon can be calculated by dividing this Energy by c (speed of light in a vacuum).
Remember photons travel at c in a vacuum. Nothing that can travel at c has mass.
In modern physics you don’t need mass to have momentum. In classical physics you do.
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