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Frequency
Frequency, also known as temporal or ordinary frequency, is the number of cycles of a repeating event or wave per second, measured in hertz (Hz).
Frequency comb
A frequency comb, also called an optical frequency comb, is a laser whose evenly spaced spectral lines let optical frequencies be measured with atomic-clock accuracy, work honored with the 2005 Nobel Prize in Physics.
Frequency domain
The frequency domain, also known as the Fourier domain or frequency space, is the domain in which a signal is expressed by its frequency content rather than its time content.
Frequency domain decomposition
Frequency domain decomposition (FDD) is an operational modal analysis method that extracts natural frequencies, mode shapes, and damping ratios from a structure's ambient vibration without knowing the exciting forces.
Frequency response
Frequency response, also called the frequency response function, is the description of how a linear system alters the magnitude and phase of sinusoidal inputs as frequency varies.
Frequency-bin entanglement
Frequency-bin entanglement is a form of quantum entanglement where photon pairs share correlations across discrete frequency bins rather than polarization or paths, used for fiber-based quantum key distribution.
Frequency-comb spectroscopy
Frequency-comb spectroscopy uses a laser frequency comb, a spectrum of precisely known equidistant lines, as the light source for broadband, high-resolution measurement of atomic and molecular spectra.
Frequency-dependent squeezing
Frequency-dependent squeezing is a quantum-optics technique that rotates the squeezing angle of injected squeezed light with frequency to cut quantum noise across a gravitational-wave detector's whole band.
Frequency-dependent squeezing in gravitational-wave detectors
Frequency-dependent squeezing is a quantum-optical technique used in LIGO and Virgo that reduces both shot noise at high frequencies and radiation pressure noise at low frequencies.
Frequency-modulated continuous-wave
Frequency-modulated continuous-wave (FMCW) LiDAR is a coherent laser ranging technique that sweeps a continuous laser's frequency, measuring both distance and velocity from the beat signal.
Frequency-modulation atomic force microscopy
Frequency-modulation atomic force microscopy (FM-AFM) is a dynamic AFM mode, introduced in 1991, that measures tip-sample forces through resonance-frequency shifts and achieves true atomic resolution in vacuum and liquid.
Frequency-resolved optical gating
Frequency-resolved optical gating (FROG) is an ultrafast optics technique that measures the full intensity and phase of an ultrashort laser pulse, replacing rough autocorrelation estimates with complete characterization.
Frequentist probability
Frequentist probability, also called frequentism, interprets probability as the long-run relative frequency of an event in repeated trials, underpinning the statistical methods of Fisher, Neyman, and Pearson.
Freshwater Biology (journal)
Freshwater Biology is a peer-reviewed scientific journal published monthly by John Wiley & Sons since 1971, covering freshwater ecology from rivers and lakes to groundwaters and wetlands.
Fresnel diffraction
Fresnel diffraction is the near-field diffraction of light that applies when source and observation distances are comparable to the aperture size; its applications include Fresnel zone plates, which focus X-rays.
Fresnel equations
The Fresnel equations, or Fresnel coefficients, are formulas giving how much light reflects and transmits at the interface between two media, developed by Augustin-Jean Fresnel in 1821–1823.
Fresnel integral
The Fresnel integrals, also called the Fresnel functions, are two special functions C(z) and S(z) that arise in Fresnel near-field diffraction, where wavelet phase varies quadratically with position.
Fresnel lens
The Fresnel lens, also called the bull's eye lens, is a thin lens of stepped concentric zones that focuses light like a much thicker lens.
Fresnel number
The Fresnel number is a dimensionless quantity in optics, F = a²/(λL), counting the Fresnel half-wavelength zones in an aperture and distinguishing Fresnel near-field from Fraunhofer far-field diffraction.
Fresnel zone
A Fresnel zone is one of a family of concentric regions, ellipsoids around a radio path or rings on a wavefront, defined by half-wavelength path differences.
Freundlich adsorption isotherm
The Freundlich adsorption isotherm is an empirical power-law equation relating the amount of a substance adsorbed per unit mass of adsorbent to its equilibrium concentration, introduced by Herbert Freundlich in 1906.
Freysteinn Sigmundsson
Freysteinn Sigmundsson (born 1966) is an Icelandic geophysicist at the University of Iceland who studies volcano geodesy, tracking magma movement, and led a Nature paper on the 2021 Fagradalsfjall eruption.
Fricative
A fricative is a consonant produced by forcing air through a narrow constriction so the airflow becomes turbulent, as in English f, s, and sh.
Friction
Friction, also called frictional force, is the force that opposes relative motion between surfaces in contact. Its empirical laws were stated by da Vinci, Amontons, and Coulomb.
Friction-induced vibration (squeal)
Friction-induced vibration, known as squeal, is a self-excited vibration in which sliding friction feeds energy into the resonant modes of sliding bodies, producing intense noise in brakes, railway wheels, and clutches.
Friday
Friday is the day of the week between Thursday and Saturday, the fifth day under ISO 8601, named for the goddess Frigga and holy in Islam and Christianity.
Fridays for Future
Fridays for Future (FFF) is a youth-led global climate strike movement begun in August 2018 by Swedish student Greta Thunberg, striking outside the Swedish parliament.
Friedberg–Muchnik theorem
The Friedberg–Muchnik theorem, proved independently by Richard Friedberg and Andrei Muchnik in 1956–1957, is a computability theory result stating that there exist two computably enumerable sets whose Turing degrees are incomparable.
Friedel–Crafts acylation
Friedel–Crafts acylation is a named organic reaction, discovered by Charles Friedel and James Crafts in 1877, that installs an acyl group onto an aromatic ring to give an aryl ketone.
Friedel–Crafts alkylation
Friedel–Crafts alkylation is a Lewis acid–catalyzed reaction attaching an alkyl group to an aromatic ring, classically with an alkyl chloride and AlCl₃, and remains widely used in industrial synthesis.