Photons are massless particles, meaning they have no rest mass. However, they do carry energy and momentum, and according to Einstein's famous equation E=mc², energy and mass are equivalent.

Therefore, photons have a relativistic mass, which is determined by their energy:

* Relativistic mass (m) = Energy (E) / Speed of light squared (c²)

This means that a photon's mass increases with its energy. While a photon has no rest mass, it's important to understand that it does carry energy and momentum, contributing to the overall mass-energy of a system.

However, it's important to note that the concept of "relativistic mass" is often considered outdated in modern physics. Physicists now prefer to use the term "energy-momentum four-vector" to describe the properties of a particle in motion.