Glossary · Vật lý
Photon
The particle of light: a quantum of electromagnetic energy. A photon always moves at the speed of light and carries an energy E = hf that depends only on its frequency f, where h is Planck's constant.
Light travels as a wave yet exchanges energy in discrete packets. Each packet is a photon. Its energy does not depend on how bright the source is, only on frequency: E = hf. Making a source brighter increases the NUMBER of photons, not the energy each one carries.
This is what the photoelectric effect forced physics to accept. Every metal has its own cut-off frequency, and below it no electrons come out however intense the light. For a metal whose cut-off falls between red and violet, very intense red light still ejects no electrons while much fainter violet light does, because each violet photon carries enough energy and each red one does not, however many of them arrive.
A photon has no rest mass and therefore cannot be at rest: in vacuum it always travels at exactly the speed of light. All electromagnetic radiation, from radio waves to gamma rays, is a stream of photons, differing only in frequency and so in the energy of each quantum.