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A **photon** is a particle of light
(or of any electromagnetic radiation), i.e., a name for
such a particle, when light is considered a stream of particles.
The particle represents the quantum of light, the minimum possible amount.
The photon theory, i.e., the modern particle theory of light,
posits the minimum possible
amount of light depends upon the light's wave frequency.
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.

According to current particle physics, photons are elementary
particles (a type of **boson**) that interact with other particles.
They naturally travel "the speed of light", which in a vacuum
is the well-known constant, and even when not in a vacuum, they
are merely slowed yet still moving.
As implied above, a photon has an associated "light-wave frequency"
and carries an amount of energy proportional to that frequency
(**photon energy**, typically measured in **electron volts**).
They have no mass (i.e., no **rest mass**), which semantically
follows from the fact that they "cannot be at rest", but some effects
of rest mass can be tested (detecting velocity dispersion in a
vacuum or its equivalent) and are indeed checked when opportunities
present themselves (astronomical phenomena producing measurable
dispersion), basically checking the theory of relativity

Phenomena successfully modeled only using photon theory include the interactions of light with atoms and electrons, such as absorption, emission and scattering. The first such instance explained by photon theory was the photoelectric effect, i.e., that the emission of electrons which can occur when EMR strikes a metal surface ("metal" as in copper, silver, etc.) does not depend upon the total light energy striking the surface, which only happens with a sufficient EMR frequency, can be explained if light arrives in quanta based upon frequency, and only frequencies associated with sufficient energy to free an such an electron succeed in doing so. Einstein suggested this in one of his 1905 papers.

https://en.wikipedia.org/wiki/Photon

absorption

ACIS

advection dominated accretion flow (ADAF)

air shower

antimatter

astronomical quantities

atmospheric escape

atomic excitation

Auger effect

Balmer jump (BJ)

baryon acoustic oscillations (BAO)

baryon

baryonic matter

black hole shadow

biofluorescence

Bohr model

Bose-Einstein statistics

bremsstrahlung

carbon (C)

Cherenkov detector

cosmic microwave background (CMB)

CMB anisotropies

CMB lensing

CMB polarization

cosmic neutrino background (CNB)

CNO cycle

color-magnitude diagram (CMD)

column density

Compton reflection

Compton scattering

Compton telescope

continuous absorption

continuum emission

cooling function

corona

cosmic rays (CR)

Cherenkov Telescope Array (CTA)

CTIS

dark matter

dark matter annihilation

de Broglie wavelength

decoupling

diffusion damping

Eddington approximation

electron degeneracy

electron orbital

electron scattering

electron shell

emission

epoch of reionization (EOR)

escape fraction

electron volt (eV)

fluorescence

free streaming

frequency

Geiger-avalanche photodiode (G-APD)

giant star

graviton

GZK limit

hardness

High-altitude Water Cherenkov Observatory (HAWC)

helium 1083 nm line

high-energy astrophysics (HEA)

helium (He)

ionized hydrogen (HII)

HII region (HII)

hydrogen (H)

integration time

ionizing radiation

KID

Klein-Nishina formula

K-line

line blanketing

Lyman series (L)

Lyman-Werner photon

mean free path

microcalorimeter

microchannel array

Milagro

natural broadening

neutrino (ν)

NICER

observable universe

optical depth (τ)

oscillator strength

pair production

pair telescope

particle

photodissociation region (PDR)

Penrose Compton scattering (PCS)

photochemistry

photodisintegration

photodissociation

photoionization

photometer

photometry

photon counting

photon energy

photon noise

photon sphere

photosphere

Planck constant (h)

photomultiplier tube (PMT)

planetary nebula (PN)

Poynting-Robertson effect

Poynting vector (S)

pressure integral

pulsar (PSR)

quantum efficiency (QE)

quantum

quantum mechanics (QM)

quantum number

quark

radiation zone

radiolysis

random walk

Rayleigh scattering

recombination

relic

radiation hydrodynamics (RHD)

Rosseland mean opacity

equation of radiative transfer (RTE)

Southern African Large Telescope (SALT)

scattering

scintillator

spectral energy distribution (SED)

self-absorption

superluminous supernova (SLSN)

spectral power distribution (SPD)

spectral line designation

spin (m

synchrotron self-Compton (SSC)

standard model

Stark effect

state of excitation

superconducting tunnel junction (STJ)

Strömgren sphere

supersymmetry (SUSY)

surface of last scattering

synchrotron radiation

Sunyaev-Zel'dovich effect (SZ effect)

TAMBO

thermal bremsstrahlung

thermal emission

thermodynamic equilibrium (TE)

Thomson scattering

tired light

two-stream approximation

ultra-high-energy gamma rays (UHEGR)

Uhuru

Heisenberg uncertainty principle

VHE

very-high-energy gamma rays (VHEGR)

vegetation red edge (VRE)

wavelength

wave-particle duality

XMM-Newton

X-ray

Zeeman effect