(quantum of light, i.e., light as a particle)
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.
advection dominated accretion flow (ADAF)
baryon acoustic oscillations (BAO)
cosmic microwave background (CMB)
cosmic rays (CR)
Cherenkov Telescope Array (CTA)
dark matter annihilation
de Broglie wavelength
epoch of reionization (EOR)
High-altitude Water Cherenkov Observatory (HAWC)
ionized hydrogen (HII)
HII region (HII)
mean free path
optical depth (τ)
photodissociation region (PDR)
Planck constant (h)
planetary nebula (PN)
Poynting vector (S)
quantum mechanics (QM)
equation of radiative transfer (RTE)
spectral energy distribution (SED)
superluminous supernova (SLSN)
spectral power distribution (SPD)
synchrotron self-Compton (SSC)
state of excitation
Sunyaev-Zel'dovich effect (SZ effect)
thermodynamic equilibrium (TE)
ultra-high-energy gamma rays (UHEGR)
very-high-energy gamma rays (VHEGR)
vegetation red edge (VRE)