Browse All 842 Physics Terms A–Z

Finite Element Method (FEM) Mathematical & Computational Physics Numerical Mesh Method A numerical technique for solving complex partial differential equations by subdividing a large physical domain into smaller, simpler finite sub-domains called elements. K u = f (Global Stiffness Matrix Equation)

Finite Potential Well Quantum & Atomic Physics eV or Joules A quantum potential well model with barriers of finite energy height V₀. V(x) = V₀ for |x| > a, V(x) = 0 for |x| ≤ a

First Law of Thermodynamics Thermodynamics & Statistical Physics J A statement of universal energy conservation applied to thermodynamic systems, asserting that internal energy change equals net heat added minus work done by the system. ΔU = Q - W

Fluorescence Quantum & Atomic Physics Nanoseconds (Lifetime) The rapid absorption of light at a shorter wavelength followed by immediate emission of light at a longer wavelength. h ν_emitted < h ν_absorbed (Stokes Shift)

Fluorescence Microscopy Biophysics & Medical Physics nm (Spatial Resolution) An optical microscopy technique utilizing fluorophores excited by specific light wavelengths to image cellular structures with high contrast. Abbe Diffraction Limit: d_min = λ / (2 NA) ≈ 200 nm

Fluoroscopy Biophysics & Medical Physics Frames per Second (fps) & Dose Rate (mGy/min) A dynamic X-ray imaging technique obtaining real-time moving video images of interior anatomical structures and contrast agents. Real-time continuous low-dose X-ray video acquisition

Fokker-Planck Equation for Plasmas Plasma Physics Kinetic Collision Operator The kinetic equation describing the time evolution of a plasma velocity distribution function subject to small-angle Coulomb collisions. (∂f / ∂t)_coll = -∇_v · (A f) + ½ ∇_v ∇_v : (B f)

Force Classical Mechanics N (kg·m/s²) A vector interaction that, when unopposed, alters the state of motion or deforms a physical body. F = m a = dp/dt

Forced Oscillations Waves, Oscillations & Acoustics m Oscillations produced in a system subjected to a periodic external driving force. m (d²x/dt²) + b (dx/dt) + k x = F0 cos(ω t)

Fourier Series Mathematical & Computational Physics Harmonic Series Expansion An expansion of a periodic function f(x) into an infinite sum of orthogonal sine and cosine harmonic functions. f(x) = a₀/2 + ∑_(n=1)^(∞) [a_n cos(n x) + b_n sin(n x)]

Fourier Transform Mathematical & Computational Physics Inverse Domain Unit (e.g. s⁻¹ or m⁻¹) An integral transform that decomposes a continuous time or spatial signal into its constituent frequency or wavevector spectrum. f̂(k) = ∫_(-∞)^(∞) f(x) e^(-i k x) dx, f(x) = (1 / 2π) ∫_(-∞)^(∞) f̂(k) e^(i k x) dk

Frame of Reference Foundations of Physics N/A A coordinate system along with a clock used to record positions, velocities, and times of physical events.

Franck-Hertz Experiment Quantum & Atomic Physics eV (Electron-Volts) An influential 1914 physics experiment that directly demonstrated the existence of quantized discrete energy states in mercury atoms. E_loss = h ν = 4.9 eV (for Mercury vapor)

Fraunhofer Diffraction Optics m Far-field wave diffraction where both incident and observed wavefronts are planar, achieved using lenses or large distances. a sin θ = m λ

Fraunhofer Lines Optics nm A set of dark absorption lines in the solar spectrum caused by chemical element absorption in the Sun's outer atmosphere. E = h ν = E_upper - E_lower

Frequency Waves, Oscillations & Acoustics Hz (s⁻¹) The number of complete wave cycles or vibrational repetitions occurring per unit time. f = 1 / T = v / λ

Frequency (Mechanics) Classical Mechanics Hz (s⁻¹) The number of complete vibrational cycles occurring per unit time. f = 1 / T = ω / (2π)

Fresnel Diffraction Optics N/A Near-field wave diffraction where source or observation screen is close to the diffracting aperture, requiring spherical wavefront curvature analysis. Fresnel Number F = a² / (λ L) ≳ 1

Friction Classical Mechanics N A resistive contact force that opposes relative sliding motion or tendency of motion between surfaces. fs ≤ μs N, fk = μk N

Fundamental Constants Foundations of Physics Various Universal physical quantities whose numerical values are constant in space and time throughout the cosmos.

Fundamental Frequency Waves, Oscillations & Acoustics Hz The lowest natural frequency at which a standing wave pattern can be established in a vibrating system. f1 = v / (2 L) (String / Open Pipe)

Galaxy Astrophysics, Cosmology & Relativity Solar Masses (M_☉) and Kiloparsecs (kpc) A massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter. M_galaxy = v² r / G (includes Dark Matter)

Galaxy Cluster Astrophysics, Cosmology & Relativity Megaparsecs (Mpc) & M_☉ A gravitationally bound structure containing hundreds to thousands of individual galaxies bound together by gravity. M_total ~ 10¹⁴–10¹⁵ M_☉ (80% Dark Matter, 15% Intracluster Gas, 5% Stars)

Gamma Decay Nuclear & Particle Physics MeV or keV (Photon Energy) A radioactive decay process where an excited atomic nucleus transitions to a lower energy state by emitting a high-energy gamma-ray photon. ᴬ_Z X* → ᴬ_Z X + γ (High-energy photon emission)

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