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A **photon** is a particle of light (electromagnetic radiation), i.e., what you call
such a particle if light is considered a stream of particles. It
is the quantum of light, the minimum possible amount.
The photon theory, i.e., the modern particle theory of light,
is that the minimum amount of light depends upon the light's
frequency, being the light-energy proportional to it.
Dating from prior to photon theory, there has been a very successful
theory that EMR consists of waves, as summarized by Maxwell's equations.
Photon theory coexists because it is successful in modeling
some light/matter interactions that the wave theory does not.

Photons are elementary particles (a type of **boson**) according to
current particle physics, interacting with other particles. They
naturally travel "the speed of light". As implied above, each
photon has an assigned frequency and carries an amount of energy
proportional to it.

Phenomena successfully modeled with photon theory includes the interaction of light with atoms and electrons, such as absorption, emission and scattering. The first such instance for photon theory was photoionization, i.e., that ionization by incoming EMR does not simply depend upon the total light energy striking the atoms, but more basically on the light's frequency, which Einstein explained as suggesting only those quanta of light carrying sufficient energy to free an electron from the atom were successful ionizing such an atom.

absorption

antimatter

astronomical quantities

atomic excitation

Balmer jump

baryon acoustic oscillations (BAO)

baryon

biofluorescence

Bohr model

Bose-Einstein statistics

bremsstrahlung

cosmic microwave background (CMB)

CMB lensing

CMB polarization

CNO cycle

column density

Compton scattering

continuous absorption

continuum emission

cooling function

cosmic rays (CR)

Cherenkov Telescope Array (CTA)

dark matter

dark matter annihilation

de Broglie wavelength

Doppler broadening

electron degeneracy

electron orbital

electron scattering

emission

epoch of reionization (EOR)

escape fraction

frequency

gamma rays

High-altitude Water Cherenkov Observatory (HAWC)

hydrogen (H)

line blanketing

Lyman-Werner photon

mean free path

Milagro

neutrino (ν)

optical depth (τ)

oscillator strength

pair production

particle

photochemistry

photodissociation

photoionization

photometer

photon counting

photon noise

photosphere

Planck constant (h)

power law

Poynting-Robertson effect

Poynting vector (S)

pressure integral

pulsar (PSR)

quantum mechanics (QM)

quantum number

random walk

recombination

equation of radiative transfer (RTE)

spectral energy distribution (SED)

superluminous supernova (SLSN)

spectral power distribution (SPD)

synchrotron self-Compton (SSC)

standard model

state of excitation

Strömgren sphere

supersymmetry

synchrotron radiation

Sunyaev-Zel'dovich effect (SZ Effect)

thermodynamic equilibrium (TE)

Thomson scattering

ultra-high-energy gamma rays (UHEGR)

very-high-energy gamma rays (VHEGR)

vegetation red edge (VRE)

wavelength

wave-particle duality

XMM-Newton