THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
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.
Optical fiber
A thin glass fibre of core + cladding structure that guides light by total internal reflection along its length. Loss < 0.2 dB/km at 1.55 μm in silica single-mode fibre; the backbone of global telecommunications.
p-polarization
An EM wave incident on an interface with its electric field parallel to the plane of incidence (German parallel). Also called TM (transverse magnetic) polarisation. Reflection coefficient passes through zero at Brewster's angle.
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.
parabola
Conic section given by a quadratic in one variable; the trajectory of a projectile under gravity alone.
parallel-axis theorem
For any axis parallel to one through the centre of mass, I = I_CM + M·d² (also called Steiner's theorem).
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.
Pascal
The SI unit of pressure: 1 Pa = 1 N/m². Named for Blaise Pascal.
pendulum clock
Mechanical clock regulated by a swinging pendulum; first accurate timekeeper, built by Huygens in 1656.
Permittivity
The constant ε in D = εE that characterises how a medium permits the establishment of an electric field. SI unit: farad per metre.
Perpendicular axis theorem
For a planar body, I_z = I_x + I_y — the moment about an axis perpendicular to the plane equals the sum of moments about two in-plane axes.
phase portrait
Plot of position versus velocity showing the trajectory of a dynamical system.
Phase space
The 2N-dimensional space of (position, momentum) pairs in which every classical state is a single point.
Phase velocity
The speed v_p = ω/k at which an individual crest of a sinusoidal wave moves. Can exceed c; carries no information.
Phasor
A complex number representing the amplitude and phase of a sinusoidal quantity. Turns linear differential equations for AC circuits into algebraic equations: V = IZ in the frequency domain.
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.
Plane wave
An EM wave whose phase is constant on planes perpendicular to the propagation direction k. Written E(r,t) = E₀ cos(k·r − ωt + φ), with ω = c|k| in vacuum. The simplest solution of the wave equation.
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).
Poisson bracket
The antisymmetric bilinear {f, g} = Σ (∂f/∂q·∂g/∂p − ∂f/∂p·∂g/∂q). Every observable evolves as df/dt = {f, H}.
Polarization
The alignment of bound charges inside a dielectric — every atom or molecule turns into a tiny dipole that points along the local electric field.
Polarization axis
For a linearly polarised wave, the direction along which E oscillates. For a polariser, the transmission axis along which the incident E-component is passed. Set by the vector structure of the wave, not its scalar amplitude.
Polarization density
The vector P = (dipole moment)/(volume), measured in coulombs per square metre, that summarises how strongly a dielectric is polarized at each point.
potential energy
Energy stored in the configuration of a system against a conservative force, retrievable by reversing that configuration.
potential well
Region of potential energy that traps a system; shape determines oscillation character.
power
The rate at which work is done or energy is transferred: P = dW/dt, measured in watts (J/s).
Poynting vector
S = (1/μ₀)·E×B. The vector whose magnitude gives the energy-flux density (W/m²) of the electromagnetic field and whose direction gives the flow direction. Introduced by Poynting in 1884.
Poynting's theorem
∂u/∂t + ∇·S = −J·E. The local statement of energy conservation for the electromagnetic field: rate of change of field-energy density plus divergence of energy flux equals the negative of work done by fields on charges.
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.