According to Einstein's theory of relativity, the rest mass of a photon is zero. This means that a photon has no mass when it is at rest. However, a photon does have energy, and its energy is proportional to its frequency. The higher the frequency of a photon, the more energy it has.

The rest mass of a photon is zero because it is a massless particle. This means that a photon does not have any mass, even when it is at rest. This is in contrast to other particles, such as electrons and protons, which have a non-zero rest mass.

The fact that photons have no rest mass has a number of important consequences. For example, it means that photons cannot be accelerated or decelerated. This is because acceleration and deceleration require a change in mass, and photons cannot change their mass.

Another important consequence of the fact that photons have no rest mass is that they travel at the speed of light. This is because the speed of light is the same for all massless particles.

The rest mass of a photon is zero because it is a massless particle. This has a number of important consequences, including the fact that photons cannot be accelerated or decelerated and that they travel at the speed of light.