Photons are elementary particles that belong to a class of particles called bosons. While it is true that photons have no rest mass, they do possess energy and momentum. The concept of mass in particle physics is distinct from energy and momentum.
In classical physics, mass is often associated with matter and is considered as a measure of the amount of material present in an object. However, in the realm of quantum mechanics and particle physics, mass takes on a different meaning. The mass of a particle is related to its resistance to changes in motion and its interaction with the Higgs field (a field that gives particles their masses).
Photons, being massless particles, travel at the speed of light in a vacuum. According to Einstein's theory of relativity, particles with rest mass cannot reach or exceed the speed of light, while massless particles, such as photons, always travel at the speed of light.
The energy and momentum of a photon are related to its frequency and wavelength through the equation E = hf, where E is the energy, h is Planck's constant, and f is the frequency. The momentum of a photon is given by p = hf/c, where c is the speed of light. Since photons have energy and momentum, they exhibit particle-like behavior, even though they lack rest mass.
It's worth noting that photons interact with matter through electromagnetic interactions. They can be absorbed, emitted, scattered, or create electric and magnetic fields. These interactions demonstrate the particle nature of photons. However, their lack of rest mass distinguishes them from massive particles such as electrons or protons.