THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Milankovitch cycles
The three astronomical cycles — eccentricity, obliquity, precession — that drive Earth's ice ages on 10,000- to 100,000-year timescales.
monkey and the hunter
Classic thought experiment: a dart fired straight at a monkey that falls at the trigger still hits the monkey, because both are in the same free fall.
nutation
Small oscillations of a spinning body's axis superimposed on its steady precession.
Optical dispersion
The wavelength-dependence of refractive index, n(λ). Causes prism splitting of white light into its colours and chromatic aberration in lenses. Quantified by the Abbe number or the Sellmeier dispersion equation.
Pair production
The conversion of energy into a matter-antimatter particle pair, most commonly a high-energy photon converting to an electron-positron pair near a nucleus: γ + nucleus → e⁺ + e⁻ + nucleus. Threshold ≥ 2m_e c² = 1.022 MeV; first observed by Carl Anderson in 1932 cloud-chamber tracks, confirming Dirac's 1928 antimatter prediction.
Paramagnetism
The weak alignment of permanent atomic magnetic moments with an applied field, competing against thermal randomisation. Susceptibility is positive and follows Curie's law χ = C/T.
Piezoelectricity
The appearance of an electric voltage across certain crystals when they are mechanically squeezed — and the converse: the same crystals deform when a voltage is applied.
Poincaré recurrence
In a bounded Hamiltonian system, almost every trajectory returns arbitrarily close to its starting state — given enough time.
Poiseuille flow
Steady laminar flow through a cylindrical pipe driven by a pressure drop. Volumetric flow rate Q = πR⁴Δp/(8ηL).
precession
The slow conical sweep of a spinning body's axis of rotation when a torque acts perpendicular to its angular momentum.
Precession of the equinoxes
The slow 26,000-year conical sweep of Earth's rotation axis, which makes the equinoxes drift through the zodiac.
projectile motion
Motion of an object fired into the air under gravity alone; the trajectory is a parabola in vacuum.
Radiation pressure
The mechanical pressure an electromagnetic wave exerts on a surface it strikes: I/c for absorbers, 2I/c for perfect reflectors, where I is the intensity in W/m². Discovered in principle by Maxwell (1871), measured by Lebedev (1901).
Redshift
The stretching of a wave's wavelength when source and observer move apart.
Relativistic beaming
The forward concentration of radiation from a relativistic source into a cone of half-angle ≈ 1/γ, caused by the Lorentz transformation of solid angles. Responsible for pulsar pulsed emission, blazar flux variability, and the lighthouse behaviour of synchrotron sources.
Relativistic Doppler effect
The frequency shift of light from a moving source observed in any inertial frame, including the time-dilation factor γ alongside the classical motion contribution. For a source receding at radial velocity v: f' = f √((1 − β)/(1 + β)). Reduces to the classical Doppler formula plus a γ correction.
resonance
Amplitude peak when driving frequency matches natural frequency.
Rutherford scattering
The elastic Coulomb scattering of a charged particle off a fixed point charge, following the hyperbolic trajectory of inverse-square central-force motion. Differential cross-section dσ/dΩ ∝ 1/sin⁴(θ/2). Full treatment in a later branch.
Single-slit diffraction
The intensity pattern I(θ) = I₀ sinc²(πa sin θ/λ) produced when light of wavelength λ passes through a slit of width a. First minimum at sin θ = λ/a; central maximum carries most of the energy.
Sonic boom
The sharp pressure impulse heard when the Mach cone of a supersonic source sweeps past an observer.
Synchrotron radiation
The electromagnetic radiation emitted by a relativistic charged particle following a curved trajectory in a magnetic field. Power scales as γ⁴ in circular motion; the spectrum is broad with characteristic frequency ω_c ∝ γ³c/R. Basis of synchrotron light sources and pulsar emission.
tidal force
The differential gravitational pull across an extended body; stretches along the line to the attractor and compresses perpendicular to it.
Total internal reflection (TIR)
The 100% reflection of light at an interface from a denser to a less dense medium when the angle of incidence exceeds the critical angle θ_c = arcsin(n₂/n₁). The basis of optical fibres, binoculars, and retroreflectors.
Transverse Doppler effect
The relativistic frequency shift observed when the source's velocity is purely perpendicular to the line of sight at the moment of emission. Reduces to f = f₀/γ — pure time dilation, with no classical Doppler contribution. The cleanest experimental test of time dilation.
Turbulence
Fluid motion organised into nested vortices across a vast range of scales. The last unsolved problem of classical physics.
Venturi effect
The drop in static pressure observed when a fluid flows through a constricted section of pipe.
Young's interference experiment
Thomas Young's 1801 double-slit experiment that demonstrated the wave nature of light by producing interference fringes. Full treatment in §09.7 interference and §09.8 diffraction-and-the-double-slit.