Browse All 842 Physics Terms A–Z

Length Contraction Astrophysics, Cosmology & Relativity Meters (Spatial Length) The relativistic effect where an object's length in the direction of motion is measured shorter by a stationary observer than its proper length L₀. L = L₀ / γ = L₀ √(1 - v²/c²)

Lens Maker's Formula Optics m⁻¹ (Diopters) An equation relating a thin lens's focal length to its material refractive index and the radii of curvature of its two spherical surfaces. 1/f = (n - 1) [ (1/R1) - (1/R2) ]

Lenz's Law Electromagnetism & Electronics Dimensionless (Directional Law) A physical law asserting that the direction of an induced current always produces a magnetic field that opposes the change in magnetic flux that induced it. Represented by the negative sign in E = -dΦ_B/dt

Leptons Nuclear & Particle Physics Elementary Particle Class A family of elementary spin-1/2 fermions that do not experience the strong nuclear interaction. Spin = ½, Baryon Number B = 0, Lepton Number L = 1

Light Year Astrophysics, Cosmology & Relativity Light-Years (ly) An astronomical unit of length equal to the distance light travels in a vacuum in one Julian year (365.25 days). 1 ly = c × 1 year ≈ 9.461 × 10¹⁵ m ≈ 63,241 AU

Linear Algebra Mathematical & Computational Physics Vector Spaces & Matrices The mathematical discipline studying vector spaces, linear transformations, matrices, and systems of linear equations. A x = b, v · w = ∑ v_i w_i

Linear Energy Transfer (LET) Biophysics & Medical Physics keV/μm (Kiloelectronvolts per micrometer) A measure of the rate at which an ionizing radiation particle deposits ionization energy along its path length through biological tissue. LET = dE / dx

Linear Thermal Expansion Thermodynamics & Statistical Physics K⁻¹ (°C⁻¹) The fractional change in length of a solid material per degree change in temperature. ΔL = α L0 ΔT

Liquid Crystals Condensed Matter & Materials Physics Dimensionless Orientational Order S States of matter possessing physical properties intermediate between conventional isotropic liquids and ordered crystalline solids. Nematic Order Parameter S = ½ ⟨3 cos² θ - 1⟩

Liquid Drop Model Nuclear & Particle Physics MeV (Binding Energy Calculation) A classical nuclear model treating the atomic nucleus as an incompressible liquid droplet of dense nuclear fluid bound by short-range nuclear forces. E_b = a_v A - a_s A^(2/3) - a_c Z²/A^(1/3) - a_a (A-2Z)²/A + δ

Lissajous Figures Waves, Oscillations & Acoustics N/A Complex geometric curves traced by combining two perpendicular simple harmonic oscillations. x(t) = A sin(a t + δ), y(t) = B sin(b t)

Lithosphere Geophysics & Environmental Physics km (Lithospheric Thickness) The rigid, brittle outermost mechanical shell of Earth comprising the entire crust and uppermost solid mantle. Elastic Thickness h_e ~ 10–200 km

Local Group Astrophysics, Cosmology & Relativity Megaparsecs (Mpc) The small galaxy cluster group comprising the Milky Way, Andromeda (M31), Triangulum (M33), and over 80 dwarf satellite galaxies. Diameter ~ 3 Mpc (~10 million ly), Mass ~ 2 × 10¹² M_☉

Logic Gates Electromagnetism & Electronics Binary (0 or 1) Elementary digital electronic building blocks that perform Boolean logical operations on one or more binary inputs to yield a single binary output. AND: Y = A·B; OR: Y = A + B; NOT: Y = Ā; NAND: Y = (A·B)̄

Longitudinal Wave Waves, Oscillations & Acoustics m A wave in which medium particles oscillate parallel to the direction of wave energy propagation. s(x,t) = s_max cos(k x - ω t)

Lorentz Factor Astrophysics, Cosmology & Relativity Dimensionless Factor (γ ≥ 1) The fundamental relativistic factor by which time dilation, length contraction, and relativistic mass scale for objects moving at velocity v. γ = 1 / √(1 - v²/c²) = 1 / √(1 - β²)

Lorentz Force Electromagnetism & Electronics N (Newtons) The combined force exerted on a charged particle moving through electromagnetic fields containing both electric and magnetic field components. F = q (E + v × B)

Loudness Waves, Oscillations & Acoustics Sone (or Phon) The subjective human psychological sensation of sound strength, depending on physical intensity and ear frequency response. Loudness ∝ log10(I / I0)

Luminous Flux Optics Lumens (lm) A photometric quantity measuring total perceived power of light emitted by a source, weighted by human eye spectral sensitivity. Φ_v = 683 ∫ P(λ) V(λ) dλ

Lyman Series Quantum & Atomic Physics Ultraviolet Wavelengths (nm) The hydrogen spectral series resulting from electron transitions dropping from higher excited states down to the ground state (n=1). 1 / λ = R_H (1 / 1² - 1 / n_i²), n_i = 2, 3, 4...

Mach Number Waves, Oscillations & Acoustics Dimensionless A dimensionless speed ratio equal to the speed of an object or fluid divided by local speed of sound in the surrounding medium. M = v / v_sound

Machine Learning in Physics Mathematical & Computational Physics Computational AI Framework The application of artificial intelligence, neural networks, and statistical learning models to solve complex physical equations and discover empirical patterns in experimental data. Physics-Informed Neural Networks (PINN): Loss = L_data + λ L_physics

Macrostate Thermodynamics & Statistical Physics N/A The macroscopic state of a thermodynamic system defined by observable thermodynamic variables like pressure, volume, temperature, and total energy. Defined by (P, V, T, N, U)

Magnetic Confinement Plasma Physics Dimensionless Ratio (Plasma Beta β) The trapping of high-temperature fusion plasma using intense magnetic fields that exert Lorentz forces to prevent plasma particles from contacting container walls. β = p / (B² / (2 μ₀)) = 2 μ₀ n k_B T / B²

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