A photon is a particle that carries electromagnetic radiation. It is the fundamental unit of light.
- A photon is an elementary particle, a quantum of light , so a photon is the smallest quantity (quantum, plural quanta) into which light can be divided.
- Light can exhibit both wave-like and particle-like behaviour. So at times light behaves as a wave, at others, it behaves like both waves and photons, and at others just as particles.
- Photons are the force carriers of radiant energy (electromagnetic radiation).
- Photons can be emitted or absorbed by charged particles such as electrons and protons.
- Photons carry energy and momentum, which are proportional to their frequency and wavelength respectively.
- Photons can interact with matter through processes such as scattering, absorption, and emission.
- Photons can be polarized, meaning their electric and magnetic fields oscillate in a particular direction.
- Photons do not have mass and propagate at the speed of light in a vacuum. We can break this statement down as follows:
- According to the theory of relativity, any object that has mass requires energy to accelerate.
- The amount of energy required to accelerate an object increases as the object’s mass increases.
- Photons are unique in that they have zero rest mass, which means they do not require any energy to be accelerated.
- As a result, they always move at the speed of light in a vacuum.
- So photons always travel at the speed of light and never slow down or come to a stop.
- The fact that photons have zero rest mass has important implications for the behaviour of light and electromagnetic radiation in general. It means that:
- The energy of a photon is directly proportional to its frequency (as described by Planck’s law).
- The momentum of a photon is proportional to its frequency and direction of travel (as described by Einstein’s theory of relativity).
About the properties of photons
- A photon is a type of elementary particle that is a quantum (plural = quanta) of the electromagnetic field. This means that it is the smallest quantity into which light can be divided.
- A photon carries energy and can be described both in terms of a particle and a wave.
- While the wave model of light works well for some phenomena, the particle model is necessary to explain others.
- Light can exhibit both wave-like and particle-like behaviour depending on the experiment performed. This is known as wave-particle duality.
- The wavelength of a photon determines its energy and frequency.
- Photons with longer wavelengths have lower energy and frequency, while photons with shorter wavelengths have higher energy and frequency.
- The wavelength of a photon can also affect its behaviour, such as its ability to penetrate materials or cause photochemical reactions.
Other properties of photons include:
- Photons have zero rest mass but have energy and momentum proportional to their frequency.
- Unlike other kinds of elementary particles, photons have no rest mass.
- Photons are electrically neutral, meaning they have no electric charge.
- Photons are stable particles that do not decay over time.
- Photons can interact with other particles, such as electrons, through processes such as absorption and emission.
- Photons can interact with other particles, such as electrons, through processes like absorption and emission.
- Photons always travel at the speed of light in a vacuum, regardless of their frequency or energy.
Related diagrams
Each diagram below can be viewed on its own page with a full explanation.
