Concept of Rest Mass of Photon:
According to Einstein’s quantum theory light propagates in the form of packets i.e. quanta of energy, which is called photon.
The rest mass of photons is being zero. It can be shown, according to the relativity theory of light.
According to the relativistic theory equation, the mass of the photon is computed as:
Explanation:
Rest mass of photon is zero. Photon has no existence at rest.
If you mean to an observer in quantum physics, actually an observer is any interaction between a quantum (superposition) state of one or more statistical entities called particle-wave with other particles or their fields like gravitation, which cause a collapse to the quantum state of those entities and turn them into definite particles.
