Browse All 842 Physics Terms A–Z

Range Classical Mechanics m The total horizontal distance traveled by a projectile from launch to landing at the same vertical height. R = (v0² sin 2θ) / g

Rayleigh Criterion Optics rad (radians) The condition for two point sources to be barely resolved by an optical instrument, stating that the central diffraction maximum of one image coincides with the first minimum of the other. θ_min = 1.22 λ / D

Rayleigh Scattering Optics W/m² The elastic scattering of light or electromagnetic radiation by particles much smaller than the light's wavelength. I_scattered ∝ 1 / λ⁴

RC Circuit & Time Constant Electromagnetism & Electronics s (Seconds) An electrical circuit composed of resistors and capacitors driven by a voltage source, characterized by its exponential charging and discharging time constant τ. V(t) = V₀ e^(-t / τ), τ = R C

Real Image Optics m An optical image formed by actual convergence of light rays, which can be projected onto a physical screen. v > 0 (for real image), m < 0 (inverted)

Reciprocal Lattice Condensed Matter & Materials Physics m⁻¹ (Reciprocal Distance) The Fourier transform space of a real-space crystal lattice, where every point corresponds to a set of parallel lattice planes in the real crystal. b₁ = 2π (a₂ × a₃) / (a₁ · (a₂ × a₃)), G · R = 2π n

Red Giant Astrophysics, Cosmology & Relativity Solar Radii (R_☉) & Kelvin A large, luminous star in a late evolutionary stage that has exhausted core hydrogen fuel and expanded its outer envelope. Radius R ~ 10–100 R_☉, T_surface ~ 3,000–4,500 K

Reflection Optics Degrees (°) The bouncing back of a light ray or wave when it strikes a boundary surface between two different media without entering the second medium. θ_i = θ_r

Refraction Optics Dimensionless (n) The bending of a light ray as it passes obliquely from one transparent medium into another with a different refractive index. n1 sin θ1 = n2 sin θ2 (Snell's Law)

Refrigerator / Heat Pump Thermodynamics & Statistical Physics Dimensionless (Coefficient of Performance) A thermodynamic device operating in reverse that consumes mechanical work to extract heat from a cold region and reject it into a warmer region. COP_ref = Q_cold / W, COP_hp = Q_hot / W

Relative Biological Effectiveness (RBE) Biophysics & Medical Physics Dimensionless Ratio The ratio of absorbed dose of a reference 250 kVp X-ray beam to the absorbed dose of a test radiation type required to produce an identical biological effect. RBE = D_reference (250 kVp X-rays) / D_test (for equal biological effect)

Relative Humidity Geophysics & Environmental Physics Percentage (%) The ratio of the actual vapor pressure of water in an air parcel to the saturation vapor pressure at the same temperature, expressed as a percentage. RH = (e / e_s(T)) × 100%

Relativistic Kinetic Energy Astrophysics, Cosmology & Relativity Joules or MeV / GeV The work required to accelerate a massive particle from rest to a relativistic velocity v. E_k = (γ - 1) m₀ c² = E_total - m₀ c²

Relativistic Momentum Astrophysics, Cosmology & Relativity kg·m/s or GeV/c The linear momentum of an object moving at relativistic speeds, scaled by the Lorentz factor γ. p = γ m₀ v = m₀ v / √(1 - v²/c²)

Relativistic Plasma Plasma Physics keV or MeV (Thermal Energy) A high-energy density plasma where thermal electron energies approach or exceed rest mass energy (511 keV), requiring relativistic plasma dynamics. k_B T_e ≳ m_e c² ≈ 511 keV, γ_e ≫ 1

Relativistic Time Dilation Astrophysics, Cosmology & Relativity Seconds (Time Interval) The phenomenon where time passes slower for a clock in relative motion compared to a stationary observer's reference clock. Δt = γ Δt₀ = Δt₀ / √(1 - v²/c²)

Remote Sensing in Geophysics Geophysics & Environmental Physics Multi-Spectral Satellite Imagery The acquisition of geophysical information about Earth's surface and atmosphere from a distance using satellite or airborne sensors. NDVI = (NIR - Red) / (NIR + Red)

Renormalization Nuclear & Particle Physics Theoretical Technique A mathematical technique in quantum field theory used to handle divergent infinite loop integrals by re-expressing parameters in terms of observed physical values. m_obs = m_bare + δm, e_obs = e_bare + δe

Reproducibility Foundations of Physics N/A The requirement that an experimental finding can be independently duplicated by other researchers using the same procedures.

Resolving Power Optics Dimensionless The ability of an optical instrument or grating to separate and distinguish two closely spaced point objects or adjacent spectral wavelengths. R = λ / Δλ = N m (Grating), RP = 1 / Δθ

Resonance Waves, Oscillations & Acoustics Hz The phenomenon where an oscillating system driven at its natural frequency absorbs maximum energy, resulting in large-amplitude vibrations. f_driven = f_natural = (1 / 2π) √(k / m)

Restoring Force Classical Mechanics N A force that acts to bring a body back toward its stable equilibrium position whenever it is displaced. F = -k x

Resultant Vector Foundations of Physics Various The single combined vector that produces the exact same net physical effect as two or more individual vectors acting simultaneously. R = A + B = √(A² + B² + 2AB cos θ)

Reverberation Waves, Oscillations & Acoustics s The persistence of sound in an enclosed space caused by multiple continuous reflections from room boundaries after the original source stops. RT60 = 0.161 V / A (Sabine's Formula)

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