The fact that photons have no rest mass while still possessing energy is one of the intriguing aspects of particle physics. Photons are elementary particles and the carriers of electromagnetic radiation, including visible light, radio waves, X-rays, and gamma rays. According to the theory of relativity, the energy (E) of a particle is related to its momentum (p) by the equation E = pc, where c is the speed of light.
For particles with mass, such as electrons or protons, their energy can be split into two components: the rest energy, which is the energy associated with the mass of the particle when it is at rest, and the kinetic energy, which arises from its motion. However, photons do not have rest mass. Instead, their energy arises solely from their motion, or more precisely, their momentum.
The momentum of a photon is given by its wavelength (λ) and its frequency (ν) through the equation p = h/λ = E/c, where h is Planck's constant. This relation, known as the de Broglie relation, links the momentum of a particle to its wavelength. Photons have both wavelength and frequency, which correspond to their wave-like nature. Therefore, despite lacking rest mass, photons possess energy due to their momentum, which is determined by their frequency and wavelength.
The energy of a photon is directly proportional to its frequency, as stated by Planck's equation: E = hν. This equation demonstrates the dual nature of photons, as they can exhibit both wave-like and particle-like properties. While photons are massless particles, they carry energy through their electromagnetic nature and their associated momentum.