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|.
Big Bang Nucleosynthesis
The forging of the light elements in the first few minutes of the universe.
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.
Brownian motion
The incessant random jitter of a small particle suspended in a fluid, driven by molecular collisions — the visible proof that atoms are real.
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.
Cosmic Microwave Background
The relic blackbody radiation released when the universe became transparent, now cooled to 2.725 K.
Cosmological redshift
The stretching of light's wavelength by the expansion of space, not by any motion of the source.
Critical opalescence
The milky scattering of light by density fluctuations that grow to the wavelength of light near a critical point.
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.
Diffusion
The net spreading of particles from crowded to sparse regions, the macroscopic face of countless random molecular walks.
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.
Einstein Ring
The complete circle of light seen when a source, a lensing mass, and the observer line up exactly.
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.