What Is a Photon?

A photon is an elementary particle and the quantum, or discrete packet, of electromagnetic radiation. Visible light consists of photons, but so do radio waves, microwaves, infrared radiation, ultraviolet light, X-rays and gamma rays.

Photons belong to the boson family. They have no rest mass, yet they carry energy and momentum. In a vacuum, they travel at the speed of light—approximately 300,000 kilometers per second, or 186,000 miles per second.

Why Photons Behave Like Particles and Waves

One of a photon's defining characteristics is wave-particle duality. Depending on how electromagnetic radiation is produced, measured or observed, it can display properties associated with either waves or particles.

Wave behavior appears in phenomena such as interference and diffraction. Particle-like behavior becomes clear when light transfers energy in discrete amounts. In the photoelectric effect, for example, photons striking a suitable metal surface can eject electrons from it.

Albert Einstein's 1905 explanation of the photoelectric effect helped establish the idea that light energy is quantized. This work became an important part of the development of quantum physics.

Photon Energy

A photon's energy depends on the frequency of the electromagnetic radiation: higher-frequency photons carry more energy than lower-frequency photons. This is why X-ray photons are more energetic than photons of visible light, while radio-wave photons carry less energy.

Although individual photons have no rest mass, they can still interact with matter and transfer energy or momentum. Everyday examples include radio signals reaching an antenna, visible light entering the eye and microwaves transferring energy to food.

How Technology Uses Photons

Photon-based science and engineering are collectively known as photonics. The field underpins technologies that generate, control, transmit and detect light and other forms of electromagnetic radiation.

• Fiber-optic communications: Light carries encoded information through optical fibers, enabling high-capacity transmission across long distances.
• Lasers: Lasers produce highly organized light whose photons have closely matched wavelengths and phase relationships.
• Medicine: Photon-based systems support laser treatments and multiple forms of medical imaging.
• Astronomy: Telescopes and detectors collect photons to reveal the properties of distant stars, galaxies and other objects.
• Quantum computing: Photons can be used to carry and process quantum information in certain quantum technology architectures.

A Foundation of Modern Physics

The photon connects fundamental physics with practical engineering. Its lack of rest mass, ability to transport energy and momentum, and dual wave-particle behavior make it central to quantum theory. The same properties also support optical communications, lasers, imaging systems and emerging quantum technologies.