Photon, particle of light energy, or energy that is generated by moving electric charges. Energy generated by moving charges is called electromagnetic radiation. Visible light is one kind of electromagnetic radiation. Other kinds of radiation include radio waves, infrared waves, and X rays. All such radiation sometimes behaves like a wave and sometimes behaves like a particle. Scientists use the concept of a photon to describe the effects of radiation when it behaves like a particle. (See  Wave Motion)

Most photons are invisible to humans. Humans only see photons with energy levels that fall within a certain range. We describe these visible photons as visible light. Invisible photons include radio and television signals, photons that heat food in microwave ovens, the ultraviolet light that causes sunburn, and the X rays doctors use to view a person's bones.

The photon is an elementary particle, or a particle that cannot be split into anything smaller. It carries the electromagnetic force, one of the four fundamental forces of nature, between particles. The electromagnetic force occurs between charged particles or between magnetic materials and charged particles. Electrically charged particles attract or repel each other by exchanging photons back and forth.


II. Characteristics


Photons are particles with no electrical charge and no mass, but they do have energy and momentum, a property that allows photons to affect other particles when they collide with them. Photons travel at the speed of light, which is about 300,000 km/sec (about 186,000 mi/sec). Only objects without mass can travel at the speed of light. Objects with mass must travel at slower speeds, and nothing can travel at speeds faster than the speed of light.

The energy of a photon is equal to the product of a constant number called Planck's constant multiplied by the frequency, or number of vibrations per second, of the photon. Scientists write the equation for a photon's energy as E=hv, where h is Planck's Constant and v is the frequency. Photons with high frequencies, such as X rays, carry more energy than do photons with low frequencies, such as radio waves. Photons that are visible to the human eye have energy levels around one electron volt (eV) and frequencies from 10¹⁴ to 10¹⁵ Hz (hertz or cycles per second). The number 10¹⁴ is a 1 followed by 14 zeros. The frequency of visible photons corresponds to the color of their light. Photons of violet light have the highest frequencies of visible light, while photons of red light have the lowest frequencies. Gamma rays, the highest-energy photons of all, have energies in the 1 GeV range (10⁹ eV) and frequencies higher than 10¹⁸ Hz. Gamma rays are only produced in special experimental devices called particle accelerators and in outer space.

Although momentum is usually considered a property of objects with mass, photons also have momentum. Momentum determines the amount of force, or pressure, that an object exerts when it hits a surface. In classical physics, or physics that deals with the behavior of objects we encounter in everyday life, momentum is equal to the product of the mass of an object multiplied by its velocity (the combination of its speed and direction). While photons do not have mass, scientists have found that they exert extremely small amounts of pressure when they strike surfaces. Scientists have redefined momentum to include the force exerted by photons, called light pressure or radiation pressure.