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What Is A Photon?

A photon is the quantum of energy in the form of electromagnetic radiation, emitted or absorbed by matter.

What is a photon?

The photon is a type of elementary particle. According to the principles of quantum physics, it is the quantum of the electromagnetic field. It is the carrier particle of all forms of electromagnetic radiation, including:

  • Gamma rays.
  • X-rays.
  • Ultraviolet light.
  • Visible light.
  • Infrared light.
  • Microwaves.
  • Radio waves.

Photons have zero mass at rest. They always move at the speed of light in a vacuum.

The photon has spin equal to 1, and is therefore a boson; Since its mass at rest is nil, the helicity of the photon can only be 1 or -1, but not 0.

The photon is represented by the symbol γ.

Is the photon a wave or a particle?

Like all elementary particles, photons are explained by quantum mechanics. However, they exhibit wave-particle duality, simultaneously exhibiting wave and particle properties.

It behaves like a wave in phenomena such as the refraction that takes place in a lens, or in the cancellation by destructive interference of reflected waves; however, it behaves like a particle when it interacts with matter to transfer a fixed amount of energy. This energy is inversely proportional to the wavelength.

For example, a lens can refract a single photon and in the process interfere with itself like a wave. Or it can act as a particle that has a defined position and a measurable amount of motion.


The wave and quantum properties of the photon are two observable aspects of the same phenomenon.

Its nature cannot be described in terms of any mechanical model. Therefore, the representation of this dual property of light, which assumes that energy is concentrated at certain points on the wavefront, is also impossible.

The quanta in a light wave cannot be located in space; some defined physical parameters of the photon are noted.

The photon in particle physics

In the standard model of particle physics, photons and other elementary particles are described as a necessary consequence of the fact that the laws of physics have a certain symmetry in space-time. The intrinsic properties of particles, such as electric charge, mass, and spin, are determined by the properties of this gauge symmetry.

The photon concept has led to far-reaching advances in theoretical and experimental physics. For example:

  • Lasers
  • The Bose-Einstein condensate
  • Quantum field theory
  • The probabilistic interpretation of quantum mechanics.

It has been applied in photochemistry, in high resolution microscopy and in the measurement of molecular distances. Recently, photons have been studied as an element of quantum computers and for their applications in optical imagery and optical communication such as quantum cryptography.

Properties of a proton

A photon has no mass, has no electric charge, and is a stable particle.

In a vacuum, a photon has two possible polarization states. The photon is the gauge boson for electromagnetism. Therefore, all the other quantum numbers in the photon (such as the number of leptons, the number of baryons, and the flavor quantum numbers) are zero. Furthermore, the photon does not obey the Pauli exclusion principle, but instead obeys the Bose-Einstein statistics.

Photons are emitted in many natural processes. For example:

  • When a charge accelerates, it emits synchrotron radiation.
  • During a molecular, atomic, or nuclear transition to a lower energy level, photons of various energies will be emitted, ranging from radio waves to gamma rays.
  • When a particle and its corresponding antiparticle are annihilated (for example, electron-positron annihilation).
    CompositionElementary particle
    InteractionsElectromagnetic, Weak, Gravity
    TheorizedAlbert Einstein (1905)
    The name "photon" is generally attributed to Gilbert N. Lewis (1926)
    <1 × 10 −18  eV / c 2
    Average lifeStable
    Electric charge0 <1 × 10 −35  e 
    C parity−1

    What are photons used for?

    Photons have many applications in technology. For example the laser.

    The laser is an extremely important application.

    Individual photons can be detected by various methods. The classic photomultiplier tube exploits the photoelectric effect: a photon of sufficient energy hits a metal plate and releases an electron, initiating an ever-widening flood of electrons.


    Semiconductor charge coupled device chips use a similar effect: an incident photon generates a charge in a microscopic capacitor that can be detected. Other detectors, like Geiger counters, use the ability of photons to ionize the gas molecules contained in the device, causing a detectable change in the gas's conductivity.

    Engineering and chemistry

    Engineers and chemists often use it in design. They are used both to calculate the energy change resulting from photon absorption and to determine the frequency of light emitted by a given photon emission.

    For example, the emission spectrum of a gas discharge lamp can be altered by filling it with (mixtures of) gases with different electronic energy level settings.

    Under some conditions, an "energy transition" can be excited by "two" photons that individually would be insufficient. This allows for higher resolution microscopy, because the sample absorbs energy only in the spectrum where two beams of different colors overlap significantly, which can be made much smaller than the excitation volume of a single beam (see two excitation microscopy. photons). Furthermore, these photons cause less damage to the sample, since they are of lower energy.

    Molecular biology

    In some cases, two energy transitions can be coupled so that as one system absorbs a photon, another nearby system "steals" its energy and re-emits a photon of a different frequency. This is the basis of fluorescence resonance energy transfer, a technique used in molecular biology to study the interaction of suitable proteins.

    Generation of random numbers

    Several different types of hardware random number generators involve detection of individual photons.

    In one example, for each bit in the random sequence to be produced, a photon is sent to a beamsplitter. In such a situation, there are two possible outcomes of equal probability. The actual result is used to determine if the next bit in the sequence is "0" or "1".

    When did the concept of photon first appear?

    In most theories until the 17th and 18th centuries, light was considered to be made up of particles. The fact that particle models could not explain phenomena such as diffraction, refraction or birefringence of light, led René Descartes, Robert Hooke and Christian Huygens to propose wave theories for light. However, the particle models remained in force, mainly due to the influence of Isaac Newton.

    Albert Einstein

    The modern concept of the photon was gradually developed by Albert Einstein in the early 20th century. This concept was used to explain experimental observations that did not agree with the classical model of light as an electromagnetic wave.

    The photon model squared with the fact that the energy of light depended on its frequency. It explained the ability of matter and electromagnetic radiation to be in thermal equilibrium. Furthermore, the photon model also explained certain anomalous observations such as black body radiation that other physicists had attempted to explain using semi-classical models. For example, Max Planck.

    Planck model

    In Planck's model, light was described by Maxwell's equations, but material objects that emitted and absorbed light did so in discrete packets of energy. Although these semi-classical models contributed to the development of quantum mechanics, several subsequent experiments validate Einstein's hypothesis that light itself is quantized. Starting with the Compton effect.

    Acceptance of the term

    In 1926 optical physicist Frithiof Wolfers and chemist Gilbert N. Lewis coined the term "photon" for these particles.

    After Arthur H. Compton won the Nobel Prize in 1927 for his scattering studies, most scientists accepted that light quanta have an independent existence and the name of photon was accepted by these quanta.

    Photovoltaic solar energy consists of converting solar radiation into electricity. Solar radiation travels from the sun to the earth through the photons.

    Some of the photons hit the first surface of the solar panel. They penetrate into it. The semiconductor materials of the photovoltaic plates absorb them.

    The photons hit the electrons present in the atoms of the semiconductors. In this way, electrons are released from their atoms. Free electrons can travel through a conductor and generate an electric current. It is electricity.

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      Published: May 13, 2015
      Last review: May 1, 2020