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**Frequency** is the number of cycles per unit time.
Typical measurement is in **hertz**, the number of cycles 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, for EMR,
the speed of light.
The energy of a photon (**photon energy**)
is directly related to its EMR frequency:

E = h × f

- E - energy of photon.
- h - Planck constant.
- f - frequency.

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**,
a length of time.

The term **angular frequency** 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** refers to the wavenumber,
a frequency in terms of distance rather than time, e.g., "how many
waves are there at some point in time over the course of a meter",
which could be measured in units of 1/m.

http://en.wikipedia.org/wiki/Frequency

http://en.wikipedia.org/wiki/Angular_frequency

absorption line

AB system

ACIS

Atacama Cosmology Telescope (ACT)

anomalous microwave emission (AME)

anisotropy

antenna temperature

Archeops

Allen Telescope Array (ATA)

Australia Telescope Compact Array (ATCA)

AtLAST

bandwidth

beat frequency

black hole merger

black-body radiation

Bohr model

bolometer

Cambridge Catalog of Radio Sources (C)

C-BASS

Canadian Hydrogen Intensity Mapping Experiment (CHIME)

chirp

chirp mass (M

cosmic microwave background (CMB)

carbon monoxide (CO)

COMAP

Compton reflection

Compton scattering

CONCERTO

Cosmic Explorer

cyclotron radiation

data cube

de Broglie wavelength

degeneracy

velocity dispersion (σ)

dispersion measure (DM)

Doppler shift

downconverter

Earth-sized VLBI

EBEX

Effelsberg 100-m Radio Telescope

eigenmode

Einstein Telescope

electric dipole radiation

electromagnetic spectrum

electron orbital

electron shell

extremely low frequency (ELF)

electromagnetic radiation (EMR)

epicyclic frequency

electron volt (eV)

fine structure

fluorescence

flux density

frequency modulation (FM)

Fourier space

Fourier transform (FT)

fast radio burst (FRB)

galactic electron density

galaxy power spectrum

gamma rays (GR)

Green Bank Telescope (GBT)

gravitational wave (GW)

GW detection (GW)

gravitational-wave detector

gravitational wave spectrum

gravitational wave strain (h)

habitable zone (HZ)

Hadley cell

hardness

Hellings and Downs curve

heterodyne spectrometer

high frequency (HF)

infrared (IR)

interference

interferometer

ionosphere

International Pulsar Timing Array (IPTA)

irradiance

Infrared Survey Facility (IRSF)

jansky (Jy)

Klein-Nishina formula

low frequency radio astronomy

LIGO

spectral line shape

Lunar Reconnaissance Orbiter (LRO)

LSPE-SWIPE

Crab Nebula (M1)

maser

microwave

millimeter astronomy

monochromatic luminosity

multichroic feedhorn array

multi-messenger astronomy

noise-equivalent power (NEP)

Next Generation Very Large Array (ngVLA)

nanohertz gravitational waves

normal mode

Nançay Radio Telescope (NRT)

Narrabri Stellar Intensity Interferometer (NSII)

neutron star merger

observable universe

opacity (κ)

optical depth (τ)

passband

phased array

photoionization

photon

photon energy

Parkes Catalog (PKS)

Planck

Planck constant (h)

plasma frequency

power law

pulsar timing array (PTA)

precipitable water vapor (PWV)

quasi-periodic oscillation (QPO)

quad-ridge flare horn (QRFH)

quantum

radiance

radiant flux

radio

radio astronomy

radiometer equation

Rayleigh-Jeans law

red noise

RELIKT-1

radio frequency interference (RFI)

RMS astronomy

Rossby number (Ro)

Rosseland mean opacity

scintillometry

spectral energy distribution (SED)

sensitivity function (S)

Square Kilometre Array (SKA)

super low frequency (SLF)

Solar and Heliospheric Observatory (SOHO)

spectral power distribution (SPD)

specific intensity

spectral correlator

spectral density

spectral flux density (S)

spectral index (α)

spectral line designation

spectral method

SPIDER

spinning dust emission

spiral galaxy

South Pole Telescope (SPT)

Sardinia Radio Telescope (SRT)

synchrotron self-Compton (SSC)

standard model of a flare

STARE

superconducting tunnel junction (STJ)

subradio

sunspot

superradiance

synchrotron radiation

Sunyaev-Zel'dovich effect (SZ effect)

telegrapher's equations

thermal bremsstrahlung

Thomson scattering

time series analysis

tremendously low frequency (TLF)

ultra low frequency (ULF)

Virgo

very low frequency (VLF)

Very Small Array (VSA)

wavelength

wavenumber (ν)

wave-particle duality

wavefront error (WFE)

Wien approximation

Wien's displacement law

atmospheric window

X-ray