Under the photon theory of light, a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy. Photons are always in motion and, in a vacuum, have a constant speed of light to all observers, at the vacuum speed of light (more commonly just called the speed of light) of c = 2.998 x 108 m/s.
According to the photon theory of light, photons…
- move at a constant velocity, c = 2.9979 x 108 m/s (i.e. “the speed of light”), in free space.
- have zero mass and rest energy.
- carry charge and momentum, which are also related to frequency nu and wavelength lambda of the electromagnetic wave E = h nu and p = h / lambda..
- can be destroyed/created when radiation is absorbed/emitted.
- can have particles-like interactions (i.e. collisions) with electrons and other particles.
Here’s an interesting depiction of what photon is or can be.