Glossary of Key Terms¶
Definitions of the recurring physical concepts used across the 18 experiments and 7 workshops in this guide, in alphabetical order. Each entry links back to the experiment(s) where the term is most central.
A¶
Airy disk — The central bright spot of the diffraction pattern formed when light from a point source passes through a circular aperture (such as a lens); its radius sets the ultimate resolution limit of any focusing system. See Focusing by a Convex Lens.
Angular divergence (θ_div) — The full angle by which a beam spreads as it propagates; a small divergence is the defining property of a well-collimated laser beam. See Directionality & Beam Quality.
C¶
Coherence length (L_c) — The maximum path difference between two beams from the same source over which a stable interference pattern is still visible; L_c = c·τ_c. See Coherence Length & Time.
Coherence time (τ_c) — The time interval over which a light wave's phase remains predictable; inversely related to the source's spectral linewidth, τ_c ≈ 1/Δν. See Coherence Length & Time.
Compton shift (Δλ) — The increase in wavelength of a photon after scattering elastically from a free electron; depends only on the scattering angle, not on the target material or beam intensity. See Compton Scattering.
Compton wavelength (λ_C) — The fundamental length scale h/(m_ec) ≈ 2.426 pm that sets the size of the Compton shift. See Compton Scattering.
D¶
Dispersion — The variation of a material's refractive index with wavelength, responsible for a prism splitting white light into a spectrum. See Dispersion by a Prism.
Dispersive power (ω) — A dimensionless measure of how strongly a prism material separates colours, ω = (n_v − n_r)/(n_y − 1). See Dispersion by a Prism.
Doppler shift (laser) — The change in frequency of light reflected from a moving surface, proportional to the surface's velocity component along the beam; the basis of laser Doppler vibrometry. See Laser Doppler Vibrometry.
F¶
f-number (N_f) — The ratio of a lens's focal length to its aperture diameter, f/D; a smaller f-number gives a tighter focused spot but a shallower depth of field. See Focusing by a Convex Lens.
Fringe width (β) — The spacing between consecutive bright (or dark) fringes in an interference pattern, β = λD/d in Young's double-slit geometry. See Young's Double-Slit Interference.
H¶
Hologram — A recording of the interference pattern between an object beam and a coherent reference beam, encoding both amplitude and phase information so that a full three-dimensional image can be reconstructed. See Making a Hologram.
M¶
Malus's Law — I(θ) = I₀cos²θ, describing how the transmitted intensity through a polarizer varies with the angle θ between its transmission axis and the incoming light's polarization direction. See Polarization & Malus's Law.
Monochromaticity — How closely a light source approaches emitting a single, pure wavelength; quantified by the spectral purity Δλ/λ. See Monochromaticity & Spectral Purity.
O¶
Numerical Aperture (NA) — A dimensionless measure of the range of angles over which an optical fiber (or lens) can accept or emit light, NA = √(n₁²−n₂²) for a step-index fiber. See Optical Fiber NA & Attenuation.
Optical path difference (OPD) — The difference in the optical path length travelled by two beams before they recombine; the quantity that actually determines constructive or destructive interference. See Michelson Interferometer.
P¶
Photon — A discrete quantum of light carrying energy E = hν and momentum p = h/λ; the particle picture of light that explains the photoelectric effect, Compton scattering and single-photon interference. See The Photoelectric Effect, Compton Scattering, Single-Photon Double-Slit Build-up.
Probability density — In the single-photon picture of interference, the quantity that predicts where an individual photon is likely to land on the screen; accumulating many photons one-by-one reproduces the classical interference pattern. See Single-Photon Double-Slit Build-up.
R¶
Resolving power (R) — The ability of a diffraction grating to distinguish two closely spaced wavelengths, R = λ/Δλ = nN, where n is the diffraction order and N the number of illuminated slits. See Single-, Double- and N-Slit Diffraction.
S¶
Speckle (laser) — The random, grainy intensity pattern produced when coherent light scatters from an optically rough surface; its contrast and grain size encode surface roughness and illumination geometry respectively. See Laser Speckle Metrology.
Spectral purity — The ratio Δλ/λ of a source's linewidth to its central wavelength; smaller values indicate a more nearly monochromatic source. See Monochromaticity & Spectral Purity.
Stopping voltage (V₀) — The retarding voltage just sufficient to stop even the most energetic photoelectrons from reaching the collector, giving a direct measurement of their maximum kinetic energy. See The Photoelectric Effect.
T¶
Threshold frequency (ν₀) — The minimum light frequency below which no photoelectrons are emitted from a given metal, regardless of intensity — the key observation classical wave theory could not explain. See The Photoelectric Effect.
W¶
Work function (φ) — The minimum energy required to free an electron from a metal surface; the energy "cost" subtracted from each absorbed photon in Einstein's photoelectric equation. See The Photoelectric Effect.