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 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", 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 volt). 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 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.


(physics,quantum mechanics)
Further reading:
http://en.wikipedia.org/wiki/Photon

Referenced by pages:
absorption
ACIS
advection dominated accretion flow (ADAF)
air shower
antimatter
astronomical quantities
atmospheric escape
atomic excitation
Auger effect
Balmer jump
baryon acoustic oscillations (BAO)
baryon
baryonic matter
biofluorescence
Bohr model
Bose-Einstein statistics
bremsstrahlung
carbon (C)
Cherenkov detector
cosmic microwave background (CMB)
CMB anisotropies
CMB lensing
CMB polarization
CNO cycle
color-magnitude diagram (CMD)
column density
Compton reflection
Compton scattering
Compton telescope
continuous absorption
continuum emission
cooling function
cosmic rays (CR)
Cherenkov Telescope Array (CTA)
dark matter
dark matter annihilation
de Broglie wavelength
decoupling
diffusion damping
Doppler broadening
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
GZK limit
hardness
High-altitude Water Cherenkov Observatory (HAWC)
high-energy astrophysics (HEA)
ionized hydrogen (HII)
HII region (HII)
hydrodynamic escape
hydrogen (H)
integration time
ionizing radiation
Klein-Nishina formula
K-line
line blanketing
Lyman series (L)
Lyman-Werner photon
mean free path
Milagro
neutrino (ν)
NICER
optical depth (τ)
oscillator strength
pair production
pair telescope
particle
photodissociation region (PDR)
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 (ms)
synchrotron self-Compton (SSC)
standard model
state of excitation
superconducting tunnel junction (STJ)
Strömgren sphere
supersymmetry (SUSY)
surface of last scattering
synchrotron radiation
Sunyaev-Zel'dovich effect (SZ effect)
Thermal Bremsstrahlung
thermal emission
thermodynamic equilibrium (TE)
Thomson scattering
ultra-high-energy gamma rays (UHEGR)
Heisenberg uncertainty principle
VHE
very-high-energy gamma rays (VHEGR)
vegetation red edge (VRE)
wavelength
wave-particle duality
Wien's displacement law
XMM-Newton
X-ray
Zeeman effect

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