Astrophysics (Index) About

### frequency

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

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
```

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.

(EMR,measure,waves)
Further reading:
http://en.wikipedia.org/wiki/Frequency
http://en.wikipedia.org/wiki/Angular_frequency

Referenced by pages:
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 (Mc)
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

Index