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Terms starting with D

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Waves, Oscillations & Acoustics

Damped Oscillations

Oscillatory motion whose mechanical energy and amplitude decrease exponentially over time due to dissipative forces like friction or…

x(t) = A exp(-b t / 2m) cos(ω_d t + φ) m
Geophysics & Environmental Physics

Darcy's Law

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 m/s (Darcy Flux q)
Astrophysics, Cosmology & Relativity

Dark Energy

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 Cosmological Constant Λ (J/m³)
Astrophysics, Cosmology & Relativity

Dark Matter

A non-baryonic form of matter that does not absorb, reflect, or emit electromagnetic radiation, detectable solely through its gravitational…

Ω_dm ≈ 0.27 (27% of total cosmic mass-energy) M_☉ or kg (Cosmological Density)
Astrophysics, Cosmology & Relativity

Dark Matter Halo

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) kpc (Halo Radius) & M_☉ (Halo Mass)
Quantum & Atomic Physics

Davisson-Germer Experiment

A 1927 physics experiment that experimentally confirmed De Broglie's matter wave hypothesis by demonstrating electron diffraction from a…

n λ = 2 d sin θ = h / p nm (Wavelength)
Quantum & Atomic Physics

De Broglie Wavelength

The wavelength associated with a massive particle, expressing its intrinsic wave-like character.

λ = h / p = h / (m v) m or nm (Wavelength)
Plasma Physics

Debye Length

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)) m (Meters)
Nuclear & Particle Physics

Decay Constant

The probability per unit time that an individual radioactive nucleus will undergo spontaneous nuclear decay.

λ = -(1 / N) (dN / dt) = ln(2) / t_½ s⁻¹ or years⁻¹
Waves, Oscillations & Acoustics

Decibel Scale

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 dB
Geophysics & Environmental Physics

Dendrochronology

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) Annual Calendar Years
Plasma Physics

Dense Plasma Focus

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) kJ (Capacitor Energy) & Short Pulses (ns)
Foundations of Physics

Density

The mass per unit volume of a substance under specified temperature and pressure conditions.

ρ = m / V kg/m³
Foundations of Physics

Dependent Variable

The factor measured or observed in an experiment that changes in response to manipulations of the independent variable.

Various
Thermodynamics & Statistical Physics

Deposition

The direct phase transition of a gas into a solid without passing through an intermediate liquid phase.

ΔH_dep = -ΔH_sub J/kg or J/mol
Foundations of Physics

Derived Units

Units of measurement formed as algebraic combinations of the seven fundamental SI base units.

Various
Electromagnetism & Electronics

Diamagnetism

A universal weak magnetic property of matter where applied magnetic fields induce opposing atomic orbital currents that weakly repel the…

χ_m < 0 (χ_m ~ -10⁻⁵) Dimensionless
Mathematical & Computational Physics

Differential Equation

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 Mathematical Equation Class
Optics

Diffraction

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) m
Optics

Diffraction Grating

An optical element with a periodic structure of thousands of closely spaced parallel rulings or lines that disperses light into its…

d sin θ = m λ (m = 0, ±1, ±2, ...) m (slits per mm)
Foundations of Physics

Dimensional Analysis

An analytical method used to verify equation consistency and establish mathematical relationships between physical quantities by examining…

[M]^a [L]^b [T]^c Dimensionless
Mathematical & Computational Physics

Dirac Delta Function

A generalized function (distribution) that is zero everywhere except at x = 0 where it is infinite, with an integral over all space equal…

δ(x) = 0 for x ≠ 0, ∫_(-∞)^(∞) δ(x) dx = 1, ∫ f(x) δ(x - a) dx = f(a) 1 / [x Unit]
Condensed Matter & Materials Physics

Dislocation

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) nm (Burgers Vector Magnitude)
Optics

Dispersive Power

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) Dimensionless