Astrophysics (Index)About

frequency

(ν, f)
(number of cycles per unit of time, such as waves per second)

Frequency is the number of recurrences of something per unit time, e.g., cycles of something repeating, such as repeating waves. The common unit of frequency, hertz is defined as the number of cycles/recurrences per second. In astrophysics, a very common use of the term frequency is in relation to electromagnetic radiation (EMR) for the rate at which individual waves are sent or received. For such a traveling wave, the wavelength times the frequency is the wave speed:

s = λ × ν

The energy of a photon (photon energy) is directly related to its EMR frequency:

E = h × ν

Frequency, wavelength, and photon energy offer three means of quantifying the same characteristic of EMR: frequency is often cited for the longest wavelengths (radio, and sometimes for microwave), and photon energy for the shortest (X-rays and gamma rays), with wavelength commonly used for visible light and the surrounding regions of the spectrum.

The term frequency is also used for other types of waves such as gravitational waves and acoustic/sound waves and other cycling astronomical phenomena such as regularly pulsing radiation (e.g., from pulsars). In general, the reciprocal of the frequency is the period (wave period, for the frequency of a repeating wave), a length of time.

The term angular frequency (typically symbolized ω) is the frequency multiplied by 2π. If a cycle is thought of as motion around a circle, this product is the number of radians traversed per unit time. It can be thought of as a shorthand, and enough physical formulas use this product to make it a convenient value.

The term spatial frequency (aka wavenumber, for traveling waves) is the number of occurrences per unit distance rather than per unit time, e.g., "how many waves are there at some point in time over the course of a meter", which can be specified in units of 1/m.


(EMR,measure,waves)
Further reading:
https://en.wikipedia.org/wiki/Frequency
https://en.wikipedia.org/wiki/Angular_frequency
https://en.wikipedia.org/wiki/Spatial_frequency
https://astronomy.swin.edu.au/cosmos/f/Frequency
http://hyperphysics.phy-astr.gsu.edu/hbase/wavrel.html
https://quantumphysicslady.org/glossary/frequency-and-wavelength/
http://earthguide.ucsd.edu/wav/frequency.html
https://pages.uoregon.edu/soper/Light/frequency.html

Referenced by pages:
AB system
absorption line
ACIS
Allen Telescope Array (ATA)
AMS-02
anisotropy
anomalous microwave emission (AME)
antenna temperature
Archeops
Atacama Cosmology Telescope (ACT)
AtLAST
atmospheric window
Auger effect
Australia Telescope Compact Array (ATCA)
bandpass filter
bandwidth
beat frequency
black hole merger
black-body radiation
Bohr model
bolometer
C-BASS
Cambridge Catalog of Radio Sources (C)
Canadian Hydrogen Intensity Mapping Experiment (CHIME)
Chinese Pulsar Timing Array (CPTA)
chirp
chirp mass (Mc)
CO ladder
COMAP
Compton reflection
Compton scattering
CONCERTO
coronal mass ejection (CME)
Cosmic Explorer (CE)
cosmic microwave background (CMB)
cosmological time dilation
Crab Nebula (M1)
cyclotron radiation
cyclotron radiation emission spectroscopy (CRES)
data cube
de Broglie wavelength
DECIGO
degeneracy
DESHIMA
dispersion measure (DM)
Doppler shift
downconverter
Earth-sized VLBI
EBEX
Effelsberg 100-m Radio Telescope
eigenmode
Einstein Telescope (ET)
electric dipole radiation
electromagnetic radiation (EMR)
electromagnetic spectrum
electron orbital
electron shell
electron volt (eV)
emission line
epicyclic frequency
European Pulsar Timing Array (EPTA)
Event Horizon Telescope (EHT)
extreme mass ratio inspiral (EMRI)
extremely low frequency (ELF)
fast radio burst (FRB)
filter bank
fine structure
Five College Radio Astronomical Observatory (FCRAO)
flux density
Fourier space
Fourier transform (FT)
frequency modulation (FM)
G band (G)
galactic electron density
galaxy power spectrum
gamma rays (GR)
GLEAM
gravitational wave (GW)
gravitational wave spectrum
gravitational wave strain (h)
gravitational-wave detector
Green Bank Telescope (GBT)
GW detection (GW)
GYRE
habitable zone (HZ)
Hadley cell
hardness
Hellings and Downs curve
heterodyne spectrometer
high frequency (HF)
Indian Pulsar Timing Array (InPTA)
infrared (IR)
intensity interferometer
interference
interferometer
International Pulsar Timing Array (IPTA)
Interplanetary Scintillation Array (IPS Array)
ionosphere
irradiance
ITU
James Clerk Maxwell Telescope (JCMT)
jansky (Jy)
Klein-Nishina formula
LIGO
LISA
low frequency radio astronomy
LSPE-SWIPE
Lunar Reconnaissance Orbiter (LRO)
MAPS
maser
MeerKAT Pulsar Timing Array (MPTA)
MeerKLASS
microwave
millimeter astronomy
monochromatic luminosity
multi-messenger astronomy (MMA)
multichroic feedhorn array
Nançay Radio Telescope (NRT)
NANOGrav
nanohertz gravitational waves
Narrabri Stellar Intensity Interferometer (NSII)
neutron star merger
noise-equivalent power (NEP)
normal mode
opacity (κ)
optical depth (τ)
optical interferometer
Parkes Catalog (PKS)
Parkes Pulsar Timing Array (PPTA)
passband
phased array
photoionization
photon
photon energy
PIXIE
Planck
Planck constant (h)
Planck function
plasma frequency
plasma wave
power law
precipitable water vapor (PWV)
Project 8
pulsar timing array (PTA)
quad-ridge flared horn (QRFH)
quantum
quasi-periodic oscillation (QPO)
radar
radiance
radiant flux
radiative forcing (RF)
radio
radio astronomy
Radio Camera Initiative (RCI)
radio frequency interference (RFI)
radiometer equation
Rayleigh-Jeans law
red noise
reddening
relativistic beaming
RELIKT-1
RMS astronomy
Rossby number (Ro)
Rosseland mean opacity
SARAS
Sardinia Radio Telescope (SRT)
scintillometry
sensitivity function (S)
Solar and Heliospheric Observatory (SOHO)
South Pole Telescope (SPT)
spaxel
specific intensity
spectral correlator
spectral density
spectral energy distribution (SED)
spectral flux density (S)
spectral index (α)
spectral line designation
spectral line shape
spectral method
spectral power distribution (SPD)
SPIDER
spinning dust emission
spiral galaxy
Square Kilometre Array (SKA)
STARE
subradio
sunspot
Sunyaev-Zel'dovich effect (SZ effect)
super low frequency (SLF)
superconducting tunnel junction (STJ)
superradiance
synchrotron radiation
synchrotron self-Compton (SSC)
telegrapher's equations
thermal bremsstrahlung
Thomson scattering
TianQin
time series analysis
tremendously low frequency (TLF)
ultra low frequency (ULF)
velocity dispersion (σ)
very low frequency (VLF)
Very Small Array (VSA)
very-high-energy gamma rays (VHEGR)
Virgo
visible light
VLASS
wave-particle duality
wavefront error (WFE)
wavelength (λ)
wavenumber (ν)
Westerhout Radio Survey (W)
Wien approximation
Wien's displacement law
X-ray
X-ray flash (XRF)

Index