Wave Phenomena and Light – Key Vocabulary

Vocabulary flashcards covering fundamental terms and definitions from the lecture on wave phenomena and light, including Huygens’ principle, interference, diffraction, and polarisation.

Wavefront

The locus of all points in a medium that vibrate in the same phase at a given instant.

Ray (of light)

A line drawn perpendicular to a wavefront indicating the direction of wave propagation.

Beam (of light)

A collection of adjacent rays travelling in the same region of space.

Huygens’ Principle

Every point on a wavefront acts as a source of secondary wavelets; the new wavefront is the forward envelope of these wavelets.

Secondary Wavelet

A small spherical wave emitted from a point on an advancing wavefront according to Huygens’ principle.

Isotropic Medium

A medium in which wave energy is transmitted equally in all directions.

Plane Wavefront

A wavefront whose points are equidistant from the source such that, over a small region, it appears flat.

Spherical Wavefront

A wavefront that forms concentric spheres around a point source.

Cylindrical Wavefront

A wavefront produced by a line source, having the shape of concentric cylinders.

Coherent Sources

Two light sources that emit waves of identical frequency, wavelength, and constant phase difference.

Superposition Principle

When two or more waves overlap, the resultant displacement equals the vector sum of individual displacements.

Interference (of light)

Redistribution of light intensity due to superposition of coherent light waves.

Constructive Interference

Superposition that produces maximum intensity when waves arrive in phase (δ = 0, 2π, …).

Destructive Interference

Superposition that produces zero intensity when waves arrive out of phase by π, 3π, …

Phase Difference (δ)

The angular difference in phase between two waves at a point, measured in radians.

Path Difference (Δ)

The physical distance between corresponding points of two waves travelling to the same location.

Young’s Double Slit Experiment

Classic demonstration of interference using two close pinholes acting as coherent sources to produce bright and dark fringes.

Fringe

A bright or dark band on the interference pattern resulting from constructive or destructive interference.

Fringe Width (β)

The separation between two successive bright (or dark) fringes, given by β = λD/d.

Bright Fringe

Region of high intensity on the screen where constructive interference occurs (Δ = nλ).

Dark Fringe

Region of minimum intensity where destructive interference occurs (Δ = (2n+1)λ/2).

Intensity (of light wave)

Power per unit area carried by a wave, proportional to the square of its amplitude.

Amplitude (of a wave)

Maximum displacement of the wave’s oscillation from its equilibrium position.

Wavelength (λ)

Distance between successive points in phase on a wave, such as crest-to-crest.

Monochromatic Light

Light consisting of a single wavelength (and therefore a single colour).

Diffraction

Bending and spreading of light when it encounters an obstacle or narrow aperture comparable to its wavelength.

Single-Slit Diffraction

Diffraction pattern produced when coherent light passes through one narrow slit, giving a central principal maximum and diminishing secondary maxima.

Principal Maximum

The central, widest, and brightest band in a single-slit diffraction pattern.

Secondary Maxima

Weaker bright bands on either side of the principal maximum in a diffraction pattern.

Polarisation (of light)

Restriction of light vibrations to a single plane perpendicular to the direction of propagation.

Plane-Polarised Light

Light whose electric field oscillates in only one plane.

Unpolarised Light

Light in which the vibration plane changes randomly and rapidly, containing all possible vibration directions.

Polaroid

A sheet containing aligned dichroic crystals that transmits light vibrating in one plane and absorbs the perpendicular component.

Dichroic Crystal

Crystal (e.g., quinine iodosulphate) that absorbs light in one polarisation direction and transmits it in the perpendicular direction.

Polarising Angle (ip)

Angle of incidence at which reflected light becomes completely plane-polarised.

Brewster’s Law

tan ip = μ; the tangent of the polarising angle equals the refractive index of the reflecting medium.

Polarimeter

Instrument that measures optical rotation; widely used in sugar industry for quality control.

Ether (historical)

Hypothetical all-pervading medium once assumed necessary for light-wave propagation.

Rayleigh Criterion (context)

Condition for the limit of resolution related to diffraction effects (mentioned implicitly via single-slit behaviour).

Path Difference for Bright Fringe

Integer multiple of wavelength: Δ = nλ (n = 0, 1, 2, …).

Path Difference for Dark Fringe

Odd half-integral multiple of wavelength: Δ = (2n+1)λ/2.

Fringe Spacing Dependence

Fringe width increases with wavelength and screen distance but decreases with slit separation (β = λD/d).

Isotropic Scattering (concept)

Equal distribution of wave energy in all directions in an isotropic medium.