In the theory of special relativity, the rest mass of a photon is indeed considered to be zero. According to Einstein's theory, the mass of an object increases as its velocity approaches the speed of light. However, this concept does not apply to massless particles like the photon.
A photon always travels at the speed of light in a vacuum, denoted by "c" in physics. Its speed remains constant, and it does not experience an increase in mass as it moves. This means that the mass of a photon is always zero, whether it is at rest or in motion. The energy of a photon is instead determined by its frequency and wavelength according to the equation E = hf, where "E" is the energy, "h" is Planck's constant, and "f" is the frequency.
It's important to note that while the rest mass of a photon is zero, it carries momentum and energy due to its wave-particle duality. This momentum and energy contribute to various phenomena such as the photoelectric effect, electromagnetic radiation, and interactions with matter.