The concept of relativistic mass is a concept that was used in earlier formulations of relativity but has fallen out of favor in modern physics. In the current understanding of physics, particularly in the framework of special relativity, the concept of relativistic mass is no longer used.
In modern physics, including special relativity, mass is considered an intrinsic property of a particle and is often referred to as the rest mass or invariant mass. Rest mass is the mass of an object as measured in its own rest frame, meaning the frame in which the object is at rest.
Photons, which are particles of light, are unique in that they are massless particles. They do not possess rest mass. According to the theory of special relativity, the energy of a photon is given by its frequency (or wavelength) through the equation E = hf, where E is the energy of the photon, h is Planck's constant, and f is the frequency of the photon. Since photons have energy but no rest mass, they travel at the speed of light in a vacuum (c), and their energy is directly proportional to their frequency.
It's important to note that the concept of relativistic mass was once used to describe how an object's mass appears to change as its velocity approaches the speed of light. However, this concept has fallen out of use because it can lead to confusion and is not as fundamental as the concept of rest mass. In modern physics, the rest mass is considered the intrinsic property of an object, and relativistic effects, such as changes in momentum and energy, are better understood in terms of those properties rather than through the concept of relativistic mass.