Astrophysics (Index)About

photon

(quantum of light, i.e., light as a particle)

A photon is a particle of light (or of any electromagnetic radiation), i.e., a term 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 opposing theory that EMR consists of waves, which is consistent with experimental evidence and with a wave-structure inherent in 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 providing evidence in support of the theory of relativity

Phenomena apparently requiring photon-theory to explain 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 ejection 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 such an electron succeed in doing so. Einstein pointed this out in one of his 1905 papers.


(physics,quantum mechanics)
Further reading:
https://en.wikipedia.org/wiki/Photon
https://en.wiktionary.org/wiki/photon
https://astronomy.swin.edu.au/cosmos/p/Photon
http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/expar.html#c5
https://pages.uoregon.edu/soper/Light/photons.html

Referenced by pages:
absorption
ACIS
advection dominated accretion flow (ADAF)
air shower
antimatter
astronomical quantities
atmospheric escape
atomic dark matter (aDM)
atomic excitation
Auger effect
Balmer jump (BJ)
baryon
baryon acoustic oscillations (BAO)
baryonic matter
Bethe-Heitler process
biofluorescence
black hole shadow
Bohr model
Bose-Einstein statistics
bremsstrahlung
carbon (C)
Cherenkov detector
Cherenkov Telescope Array (CTA)
CMB anisotropies
CMB lensing
CMB polarization
CNO cycle
collisional broadening
color-magnitude diagram (CMD)
column density
Compton reflection
Compton scattering
Compton telescope
continuous absorption
continuum emission
cooling function
corona
cosmic microwave background (CMB)
cosmic neutrino background (CNB)
cosmic rays (CR)
CTIS
DAOPHOT
dark matter (DM)
dark matter annihilation
de Broglie wavelength
decoupling
diffusion damping
Doppler broadening
dust echo
Eddington approximation
Einstein coefficients
electromagnetic spectrum
electron (e-)
electron degeneracy
electron orbital
electron scattering
electron shell
electron volt (eV)
emission
epoch of reionization (EOR)
equation of radiative transfer (RTE)
escape fraction
fluorescence
free streaming
frequency (ν)
gas pixel detector (GPD)
Geiger-avalanche photodiode (G-APD)
giant star
graviton
GZK limit
Hanbury Brown and Twiss effect (HBT effect)
Heisenberg uncertainty principle
helium (He)
helium 1083 nm line
High-altitude Water Cherenkov Observatory (HAWC)
high-energy astrophysics (HEA)
HII region (HII)
hydrogen (H)
imaging spectrometer
integration time
ionized hydrogen (HII)
ionizing radiation
IXPE
K-line
KID
Klein-Nishina formula
Lyman break (LB)
Lyman continuum (LyC)
Lyman series (L)
Lyman-Werner photon
mean free path
microcalorimeter
microchannel array
Milagro
mixture
natural broadening
neutrino (ν)
neutrino oscillation
NICER
nuclide
optical depth (τ)
oscillator strength
pair production
pair telescope
pair-instability supernova (PISN)
particle
Penrose Compton scattering (PCS)
photochemistry
photodisintegration
photodissociation
photodissociation region (PDR)
photoionization
photometer
photometry
photomultiplier tube (PMT)
photon counting
photon energy
photon noise
photon sphere
photosphere
pion (π)
Planck constant (h)
planetary nebula (PN)
Poynting vector (S)
Poynting-Robertson effect
pressure integral
proportional counter
pulsar (PSR)
quantum
quantum efficiency (QE)
quantum mechanics (QM)
quantum number
quark
radiation era
radiation hydrodynamics (RHD)
radiation zone
radioactive decay
radioactivity
radiolysis
random walk
Rayleigh scattering
recombination
relativistic beaming
relic
Rosseland mean opacity
Sachs-Wolfe effect (SWE)
scattering
scintillator
self-absorption
Southern African Large Telescope (SALT)
spectral energy distribution (SED)
spectral line designation
spectral power distribution (SPD)
spin (ms)
standard model
Stark effect
state of excitation
stellar atmosphere
stimulated emission
Strömgren sphere
strong force
Sunyaev-Zel'dovich effect (SZ effect)
superconducting tunnel junction (STJ)
superluminous supernova (SLSN)
supersymmetry (SUSY)
surface of last scattering
synchrotron radiation
synchrotron self-Compton (SSC)
TAMBO
thermal bremsstrahlung
thermal emission
thermodynamic equilibrium (TE)
Thomson scattering
tired light
two-stream approximation
Uhuru
ultra-high-energy gamma rays (UHEGR)
vegetation red edge (VRE)
very-high-energy gamma rays (VHEGR)
VHE
wave-particle duality
wavelength (λ)
X-ray
XMM-Newton
Zeeman effect

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