Physics terms starting with R

Radiation Dosimetry Biophysics & Medical Physics Gy (Gray = J/kg) The subfield of medical physics quantifying and measuring ionizing radiation energy deposited in human tissues or phantom materials. D = dĒ / dm (Absorbed Energy per Unit Mass)

Radiative Forcing Geophysics & Environmental Physics W/m² (Watts per square meter) The net change in Earth's energy balance per unit area at the tropopause due to an environmental driver of climate change. ΔF = F_incoming - F_outgoing

Radioactive Decay Nuclear & Particle Physics Bq (Bequerels = decays/s) or Ci (Curies) The spontaneous process by which an unstable atomic nucleus loses energy by emitting ionizing radiation (particles or electromagnetic rays). N(t) = N₀ e^(-λ t), A(t) = λ N(t)

Radioactive Tracer Nuclear & Particle Physics MBq or mCi (Administered Activity) A chemical compound containing a radioisotope used to trace chemical or biological pathways through living organisms or industrial systems. A(t) = A₀ e^(-λ t) (Short half-life diagnostic radiopharmaceutical)

Radiobiology Biophysics & Medical Physics Gy (Dose Response Units) The study of the biological mechanisms and health effects of ionizing radiation acting on living cells and biological tissues. DNA Double-Strand Breaks (DSB) ~ 35–40 per Gy per Cell

Radiocarbon Dating Nuclear & Particle Physics Years Before Present (BP) A radiometric dating method that measures the residual decay activity of Carbon-14 in organic materials to determine their age up to ~50,000 years. t = -(t_½ / ln 2) ln(A(t) / A₀), t_½ = 5730 years

Radiofrequency Plasma Heating Plasma Physics MW (RF Power Input) A method of heating fusion plasma by launching high-power electromagnetic waves tuned to particle cyclotron or hybrid resonance frequencies. Resonance Matching: ω_wave = ω_ce (ECRH) or ω_wave = n ω_ci (ICRH)

Radiometric Dating in Geology Geophysics & Environmental Physics Ma or Ga (Millions / Billions of Years) A geochronological technique used to determine the absolute age of rocks and minerals by measuring radioisotope parent-to-daughter ratios. t = (1 / λ) ln(1 + D / P) (Parent P → Daughter D)

Radiopharmaceuticals Biophysics & Medical Physics MBq or mCi (Administered Activity) Pharmaceutical compounds containing radioactive isotopes administered to patients for target-specific diagnostic imaging or therapy. Targeted Radionuclide Compound: Radiotracer + Carrier Molecule

Radiotherapy Biophysics & Medical Physics Gy (Gray = J/kg) Total Prescribed Dose The clinical treatment of malignant cancer tumors using precisely targeted ionizing radiation beams to induce double-strand DNA damage in cancer cells. Linear-Quadratic Cell Survival: S = exp(-α D - β D²)

Radius of Gyration Classical Mechanics m The radial distance from a rotation axis at which total mass could be concentrated without altering the moment of inertia. k = √(I / m)

Radon Gas Migration Geophysics & Environmental Physics Bq/m³ or pCi/L (Activity Concentration) The environmental migration of radioactive Radon-222 gas from decaying uranium in rocks and soil into indoor building spaces. Decay Chain: ²³⁸U → ²²⁶Ra → ²²²Rn (Half-life = 3.82 days)

Rainbow Formation Optics Degrees (°) An optical meteorological phenomenon caused by reflection, refraction, and chromatic dispersion of sunlight inside raindrops. θ_primary ≈ 42° (Red outer, violet inner)

Random Error Foundations of Physics Various Unpredictable, non-repeatable fluctuations in experimental measurements that occur with equal probability in either direction. Standard Error = σ / √N

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

Terms starting with R