A photon is a quantum of light, an electromagnetic wave. It is the basic unit of all light. The word “photon” comes from the Greek word for light, phōs. A typical photon has an energy ranging from a few electron volts to several kiloelectron volts (keV). Its momentum and spin are both zero. Photons also have no rest mass, which means they always travel at the speed of light in a vacuum.
The properties of photons are best explained by quantum mechanics. In classical physics, electromagnetic waves are described as oscillating fields that propagate through space. Quantum mechanics shows that these fields are actually made up of discrete packets of energy called quanta. Each quantum is a photon.
When a photon interacts with matter, it can give rise to different phenomena, such as reflection, refraction, or absorption. The most common interaction is scattering, where the photon changes direction but not its wavelength or frequency. This change in direction can be used to create images, as in photography or microscopy.
The study of photons is essential to many branches of physics and engineering, including optics, lasers, and semiconductor devices such as solar cells and LEDs. Photons also play a important role in astrophysics and cosmology; for example, they are responsible for the visible light coming from stars like our Sun