Science, Tech, Math › Science What Is a Photon in Physics? Photons Are a "Bundle of Energy" Share Flipboard Email Print picturegarden, Getty Images Science Physics Physics Laws, Concepts, and Principles Quantum Physics Important Physicists Thermodynamics Cosmology & Astrophysics Chemistry Biology Geology Astronomy Weather & Climate By Andrew Zimmerman Jones Math and Physics Expert M.S., Mathematics Education, Indiana University B.A., Physics, Wabash College Andrew Zimmerman Jones is a science writer, educator, and researcher. He is the co-author of "String Theory for Dummies." our editorial process Andrew Zimmerman Jones Updated September 03, 2018 A photon is a particle of light defined as a discrete bundle (or quantum) of electromagnetic (or light) energy. Photons are always in motion and, in a vacuum (a completely empty space), have a constant speed of light to all observers. Photons travel at the vacuum speed of light (more commonly just called the speed of light) of c = 2.998 x 108 m/s. Basic Properties of Photons According to the photon theory of light, photons: behave like a particle and a wave, simultaneouslymove at a constant velocity, c = 2.9979 x 108 m/s (i.e. "the speed of light"), in empty spacehave zero mass and rest energycarry energy and momentum, which are also related to the frequency (nu) and wavelength (lamdba) of the electromagnetic wave, as expressed by the equation E = h nu and p = h / lambda.can be destroyed/created when radiation is absorbed/emitted.can have particle-like interactions (i.e. collisions) with electrons and other particles, such as in the Compton effect in which particles of light collide with atoms, causing the release of electrons. History of Photons The term photon was coined by Gilbert Lewis in 1926, though the concept of light in the form of discrete particles had been around for centuries and had been formalized in Newton's construction of the science of optics. In the 1800s, however, the wave properties of light (by which is meant electromagnetic radiation in general) became glaringly obvious and scientists had essentially thrown the particle theory of light out the window. It wasn't until Albert Einstein explained the photoelectric effect and realized that light energy had to be quantized that the particle theory returned. Wave-Particle Duality in Brief As mentioned above, light has properties of both a wave and a particle. This was an astounding discovery and is certainly outside the realm of how we normally perceive things. Billiard balls act as particles, while oceans act as waves. Photons act as both a wave and a particle all the time (even though it's common but basically incorrect, to say that it's "sometimes a wave and sometimes a particle" depending upon which features are more obvious at a given time). Just one of the effects of this wave-particle duality (or particle-wave duality) is that photons, though treated as particles, can be calculated to have frequency, wavelength, amplitude, and other properties inherent in wave mechanics. Fun Photon Facts The photon is an elementary particle, despite the fact that it has no mass. It cannot decay on its own, although the energy of the photon can transfer (or be created) upon interaction with other particles. Photons are electrically neutral and are one of the rare particles that are identical to their antiparticle, the antiphoton. Photons are spin-1 particles (making them bosons), with a spin axis that is parallel to the direction of travel (either forward or backward, depending on whether it's a "left-hand" or "right-hand" photon). This feature is what allows for polarization of light.