A photon is the quantum of all forms of electromagnetic radiation, representing the fundamental particle associated with light and other forms of radiant energy, such as radio waves, X-rays, and microwaves. These particles travel at the speed of light in a vacuum and possess zero rest mass. The energy carried by a photon is directly proportional to its frequency, as described by fundamental physical equations relating energy, frequency, and Planck's constant. In biological and technological contexts, photons are critical for energy transfer, including photosynthesis in natural systems and the functioning of various imaging and communication technologies.