(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.
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.
Referenced by pages:
advection dominated accretion flow (ADAF)
Balmer jump (BJ)
baryon acoustic oscillations (BAO)
black hole shadow
cosmic microwave background (CMB)
color-magnitude diagram (CMD)
cosmic rays (CR)
Cherenkov Telescope Array (CTA)
dark matter annihilation
de Broglie wavelength
epoch of reionization (EOR)
electron volt (eV)
Geiger-avalanche photodiode (G-APD)
High-altitude Water Cherenkov Observatory (HAWC)
high-energy astrophysics (HEA)
ionized hydrogen (HII)
HII region (HII)
Lyman series (L)
mean free path
optical depth (τ)
photodissociation region (PDR)
Planck constant (h)
photomultiplier tube (PMT)
planetary nebula (PN)
Poynting vector (S)
quantum efficiency (QE)
quantum mechanics (QM)
radiation hydrodynamics (RHD)
Rosseland mean opacity
equation of radiative transfer (RTE)
Southern African Large Telescope (SALT)
spectral energy distribution (SED)
superluminous supernova (SLSN)
spectral power distribution (SPD)
spectral line designation
synchrotron self-Compton (SSC)
state of excitation
superconducting tunnel junction (STJ)
surface of last scattering
Sunyaev-Zel'dovich effect (SZ effect)
thermodynamic equilibrium (TE)
ultra-high-energy gamma rays (UHEGR)
Heisenberg uncertainty principle
very-high-energy gamma rays (VHEGR)
vegetation red edge (VRE)
Wien's displacement law