Physics terms starting with D

Damped Oscillations Waves, Oscillations & Acoustics m Oscillatory motion whose mechanical energy and amplitude decrease exponentially over time due to dissipative forces like friction or viscosity. x(t) = A exp(-b t / 2m) cos(ω_d t + φ)

Darcy's Law Geophysics & Environmental Physics m/s (Darcy Flux q) A foundational hydrogeological equation describing the flow rate of fluid through a porous medium under a hydraulic gradient. Q = -K A (dh / dl), q = Q / A = -K ∇h

Dark Energy Astrophysics, Cosmology & Relativity Cosmological Constant Λ (J/m³) An unknown form of energy with negative pressure that permeates all space and drives the accelerated expansion of the universe. Ω_Λ ≈ 0.68, w = P / ρ ≈ -1

Dark Matter Astrophysics, Cosmology & Relativity M_☉ or kg (Cosmological Density) A non-baryonic form of matter that does not absorb, reflect, or emit electromagnetic radiation, detectable solely through its gravitational effects. Ω_dm ≈ 0.27 (27% of total cosmic mass-energy)

Dark Matter Halo Astrophysics, Cosmology & Relativity kpc (Halo Radius) & M_☉ (Halo Mass) An extended, nearly spherical region of dark matter enveloping a galaxy or galaxy cluster that extends far beyond the visible stellar disk. NFW Density Profile: ρ(r) = ρ₀ / ((r/r_s) (1 + r/r_s)²)

Davisson-Germer Experiment Quantum & Atomic Physics nm (Wavelength) A 1927 physics experiment that experimentally confirmed De Broglie's matter wave hypothesis by demonstrating electron diffraction from a nickel crystal. n λ = 2 d sin θ = h / p

De Broglie Wavelength Quantum & Atomic Physics m or nm (Wavelength) The wavelength associated with a massive particle, expressing its intrinsic wave-like character. λ = h / p = h / (m v)

Debye Length Plasma Physics m (Meters) The characteristic spatial scale over which mobile charge carriers in a plasma screen out localized electrostatic potentials. λ_D = √(ε₀ k_B T_e / (n_e e²))

Decay Constant Nuclear & Particle Physics s⁻¹ or years⁻¹ The probability per unit time that an individual radioactive nucleus will undergo spontaneous nuclear decay. λ = -(1 / N) (dN / dt) = ln(2) / t_½

Decibel Scale Waves, Oscillations & Acoustics dB A logarithmic scale used to express sound intensity or sound pressure level relative to a reference threshold. β = 10 log10(I / I0), I0 = 10⁻¹² W/m²

Dendrochronology Geophysics & Environmental Physics Annual Calendar Years The scientific discipline of dating annual tree rings to reconstruct past local climate conditions and calendar dates. Tree Ring Width w = f(Temperature, Precipitation, Soil Moisture)

Dense Plasma Focus Plasma Physics kJ (Capacitor Energy) & Short Pulses (ns) A pulsed plasma device that uses electromagnetic forces to accelerate and pinch a plasma column into a short-lived, high-density hot spot. E_k ~ 10–100 kJ (Electromagnetic Acceleration and Pinch)

Density Foundations of Physics kg/m³ The mass per unit volume of a substance under specified temperature and pressure conditions. ρ = m / V

Dependent Variable Foundations of Physics Various The factor measured or observed in an experiment that changes in response to manipulations of the independent variable.

Deposition Thermodynamics & Statistical Physics J/kg or J/mol The direct phase transition of a gas into a solid without passing through an intermediate liquid phase. ΔH_dep = -ΔH_sub

Derived Units Foundations of Physics Various Units of measurement formed as algebraic combinations of the seven fundamental SI base units.

Diamagnetism Electromagnetism & Electronics Dimensionless A universal weak magnetic property of matter where applied magnetic fields induce opposing atomic orbital currents that weakly repel the material. χ_m < 0 (χ_m ~ -10⁻⁵)

Differential Equation Mathematical & Computational Physics Mathematical Equation Class A mathematical equation relating an unknown function to its derivatives with respect to one or more independent variables. F(x, y, y', y'', ..., y^(n)) = 0, d²y/dx² + ω² y = 0

Diffraction Optics m The bending, spreading, and interference of light waves around the edges of an obstacle or through a narrow aperture. a sin θ = m λ (Single Slit Minima)

Diffraction Grating Optics m (slits per mm) An optical element with a periodic structure of thousands of closely spaced parallel rulings or lines that disperses light into its component spectral wavelengths. d sin θ = m λ (m = 0, ±1, ±2, ...)

Dimensional Analysis Foundations of Physics Dimensionless An analytical method used to verify equation consistency and establish mathematical relationships between physical quantities by examining fundamental dimensions. [M]^a [L]^b [T]^c

Dirac Delta Function Mathematical & Computational Physics 1 / [x Unit] A generalized function (distribution) that is zero everywhere except at x = 0 where it is infinite, with an integral over all space equal to 1. δ(x) = 0 for x ≠ 0, ∫_(-∞)^(∞) δ(x) dx = 1, ∫ f(x) δ(x - a) dx = f(a)

Dislocation Condensed Matter & Materials Physics nm (Burgers Vector Magnitude) A 1D line defect or irregularity in a crystal lattice where atomic planes are misaligned, governing plastic deformation in metals. Burgers Vector b (Edge: b ⊥ line; Screw: b ∥ line)

Dispersive Power Optics Dimensionless A dimensionless material property measuring a prism or lens material's ability to separate different spectral wavelengths. ω = (n_v - n_r) / (n_y - 1)

Terms starting with D