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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. It
is 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", always. As implied above,
a photon has an associated "light-wave frequency"
and carries an amount of energy proportional to that frequency.

Phenomena successfully modeled with 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.

absorption

ACIS

advection dominated accretion flow (ADAF)

antimatter

astronomical quantities

atmospheric escape

atomic excitation

Auger effect

Balmer jump

baryon acoustic oscillations (BAO)

baryon

biofluorescence

Bohr model

Bose-Einstein statistics

bremsstrahlung

carbon (C)

Cherenkov detector

cosmic microwave background (CMB)

CMB lensing

CMB polarization

CNO cycle

column density

Compton reflection

Compton scattering

continuous absorption

continuum emission

cooling function

cosmic rays (CR)

Cherenkov Telescope Array (CTA)

dark matter (DM)

dark matter annihilation

de Broglie wavelength

Doppler broadening

electron degeneracy

electron orbital

electron scattering

electron shell

emission

epoch of reionization (EOR)

escape fraction

fluorescence

frequency

gamma rays (GR)

giant star

GZK limit

High-altitude Water Cherenkov Observatory (HAWC)

ionized hydrogen (HII)

HII region (HII)

hydrodynamic escape

hydrogen (H)

integration time

ionizing radiation

K-line

line blanketing

Lyman series

Lyman-Werner photon

mean free path

Milagro

neutrino (ν)

optical depth (τ)

oscillator strength

pair production

particle

photodissociation region (PDR)

photochemistry

photodissociation

photoionization

photometer

photon counting

photon noise

photosphere

Planck constant (h)

planetary nebula (PN)

power law

Poynting-Robertson effect

Poynting vector (S)

pressure integral

pulsar (PSR)

quantum

quantum mechanics (QM)

quantum number

radiolysis

random walk

Rayleigh scattering

recombination

radiation hydrodynamics (RHD)

equation of radiative transfer (RTE)

spectral energy distribution (SED)

superluminous supernova (SLSN)

spectral power distribution (SPD)

spin (m

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)

VHE

very-high-energy gamma rays (VHEGR)

vegetation red edge (VRE)

wavelength

wave-particle duality

XMM-Newton