THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Aharonov-Bohm effect
The 1959 prediction (Yakir Aharonov and David Bohm) that quantum charged particles passing through field-free regions still acquire a measurable phase shift Φ = (q/ℏ)Φ_B from the enclosed magnetic flux. Confirmed by Chambers (1960) and decisively by Tonomura (1986). The cleanest demonstration that the EM potential is more fundamental than the field.
Anomalous dispersion
A frequency range in which dn/dω < 0 (refractive index decreases with frequency), typically near absorption resonances. Occurs when the driving frequency is above a material's resonance and below its relaxation.
axial precession
The slow conical motion of the Earth's rotation axis, 25,800-year period, driven by lunar and solar tidal torques on the equatorial bulge.
beats
Amplitude modulation from superposition of two close frequencies; f_beat = |f₁ − f₂|.
Birefringence
The property of an optically anisotropic material to have different refractive indices for different polarisations of light. Splits an unpolarised beam into two refracted rays (ordinary and extraordinary).
Blueshift
The compression of a wave's wavelength when source and observer move together.
Boost mixing of E and B
The phenomenon that under a Lorentz boost, electric and magnetic field components rotate into each other. A pure-E field in one frame becomes E and B in any boosted frame; a pure-B field becomes E and B; the two are not separate physical entities.
Bremsstrahlung
German for braking radiation — the continuous electromagnetic spectrum emitted when a charged particle decelerates in the Coulomb field of a nucleus. Produces the continuum background in X-ray tubes; cuts off sharply at the Duane-Hunt limit E_max = eU.
Capacitor charging
The exponential rise V(t) = V₀(1−e^(−t/τ)) of a capacitor's voltage as it is charged through a resistor. Half the energy delivered by the source ends up in the capacitor; half is dissipated in the resistor, regardless of R.
Chandler wobble
A 433-day periodic wobble of the Earth's rotation axis with a few-metre amplitude at the surface, due to free rigid-body precession.
Chaos
Deterministic dynamics with sensitive dependence on initial conditions — nearby trajectories diverge exponentially.
Compton shift
The wavelength shift Δλ = (h/m_e c)(1 − cos θ) of light scattered off electrons, where θ is the scattering angle. Angle-dependent, independent of the incident wavelength; first measured by Arthur Compton in 1923 with X-rays on graphite — the experiment that established photons as mechanically legitimate particles.
Constructive interference
The superposition of two or more coherent waves in phase, producing an amplitude (and intensity) greater than any single wave. Condition: path-length difference = mλ for integer m.
Destructive interference
The superposition of two or more coherent waves in anti-phase, producing an amplitude (and intensity) less than the sum. Total cancellation requires equal amplitudes and a path-length difference of (m+½)λ.
Diamagnetism
The universal property of all matter to develop a weak magnetization *opposite* to an applied field. In most materials it is swamped by para- or ferromagnetism, but in closed-shell atoms it dominates.
Diffraction (EM)
The bending of light around obstacles and the spreading of light beyond apertures, resulting from the wave nature of EM radiation. Sets the resolution limit of every imaging system at about λ/NA.
Dispersion
The dependence of wave speed on wavelength or frequency — the reason a pulse spreads and a prism makes a rainbow.
Doppler effect
The shift in observed frequency when a wave source and observer move relative to each other.
Double-slit diffraction
Young's 1801 experiment. Two coherent slits a distance d apart produce an interference pattern of bright fringes at d sin θ = mλ, modulated by the single-slit envelope of each slit's width.
Eddy current
A circulating current induced inside a bulk conductor by a changing magnetic flux. Eddy currents dissipate energy as heat, and Lenz's law ensures the force on the conductor always opposes the relative motion of source and conductor.
Electromagnetic induction
The generation of an electromotive force in a circuit whenever the magnetic flux through it changes. Discovered by Faraday and Henry in 1831, it is the foundation of every electric generator, transformer, and inductive sensor ever built.
Electromagnetic wave
A self-sustaining coupled oscillation of electric and magnetic fields that propagates through vacuum at c = 1/√(μ₀ε₀). E, B, and k are mutually perpendicular. Maxwell's synthesis identified light itself as an electromagnetic wave.
Energy cascade
Richardson's picture: energy injected at large scales is handed down, eddy by eddy, to smaller scales until viscosity dissipates it as heat.
equatorial bulge
The excess of the Earth's equatorial radius over its polar radius (about 21 km), caused by the centrifugal deformation of rotation.
Ferroelectricity
The presence of a spontaneous electric polarization in certain crystals that can be reversed by an applied field — the electric analogue of ferromagnetism.
Ferromagnetism
The spontaneous parallel alignment of atomic magnetic moments via quantum exchange interaction, producing permanent magnetization below a critical Curie temperature. The origin of magnetism in iron, nickel, cobalt, and everyday magnets.
free fall
Motion under gravity alone, with no air resistance or other forces — every object accelerates at the same rate.
Kolmogorov spectrum
E(k) ∝ ε^(2/3) k^(−5/3) — the universal inertial-range energy spectrum of fully developed turbulence.
Length contraction of current
The relativistic effect that a current-carrying lattice, viewed in the rest frame of its drift electrons, has its inter-ion spacing contracted by the Lorentz factor γ. Produces the net + charge density that explains magnetic attraction as relativistic electrostatics.
Meißner effect
The active expulsion of magnetic flux from the interior of a superconductor on cooling below T_c in an applied field. The signature that superconductivity is a distinct thermodynamic phase, not mere zero resistance.