Glossary
Wave physics glossary
Plain-English definitions for every term used across this site — 119 entries covering wave anatomy, polarization, electromagnetism, seismology, and the history of wave theory. These are the same definitions that appear when you hover a dotted-underlined word anywhere on the site.
For the full treatment of any of it, start with transverse waves, longitudinal waves, the electromagnetic spectrum or seismic waves.
A
- adiabatic
- A process fast enough that no heat escapes to the surroundings. Sound compresses air adiabatically, which is why Newton's isothermal calculation came out too low.
- adiabatic index
- The ratio of a gas's heat capacities, about 1.4 for air. Laplace added it to Newton's formula and fixed the speed of sound.
- Ampère's law
- An electric current — or, with Maxwell's correction, a changing electric field — creates a magnetic field.
- amplitude
- The maximum displacement of the medium from its rest position. Energy is proportional to amplitude squared, so doubling it quadruples the energy.
- analyser
- A second polarizing filter, used to measure or further restrict the plane of already-polarized light.
- antinode
- A point of maximum oscillation on a standing wave, sitting midway between two nodes.
B
- BB84
- The first quantum key distribution protocol, encoding bits in two different polarization bases of light.
- black hole
- An object so dense that not even light can escape its gravity.
- borehole
- A deep hole drilled into the ground. The deepest ever, Russia's Kola Superdeep Borehole, reached only about 12 km.
C
- compression
- A region of a longitudinal wave where the particles of the medium are bunched closer together.
- compression wave
- Another name for a longitudinal wave, because it travels by squeezing and stretching the medium along its path.
- constructive interference
- When two waves meet in phase, crest on crest, and their amplitudes add to make a larger wave.
- corpuscular theory
- Newton's idea that light is a stream of tiny particles. His authority kept it dominant for over a century.
- crest
- The highest point of a transverse wave, where displacement above the rest position is greatest.
- crust
- Earth's thin outermost rock layer, roughly 35 km deep beneath the continents.
D
- density
- How much mass is packed into a given volume. Sound speed balances a medium's stiffness against its density.
- destructive interference
- When a crest meets an equal trough, the two displacements cancel and the medium stays still.
- diffraction
- The spreading of a wave as it passes an edge or squeezes through a narrow opening.
- displacement
- How far a particle has moved from its rest position at a given moment.
- displacement current
- Maxwell's addition to Ampère's law: a changing electric field behaves like a current and creates a magnetic field.
- Doppler effect
- The shift in a wave's observed frequency when the source and observer move relative to each other — why a passing siren drops in pitch.
- double-slit experiment
- Thomas Young's 1801 demonstration that light through two slits makes interference fringes — strong evidence it is a wave.
E
- echolocation
- Finding prey and obstacles by emitting high-frequency sound and interpreting the echoes, as bats and dolphins do.
- elastic
- Describes a material that springs back to its original shape after being deformed.
- elastic modulus
- A measure of how strongly a material resists being deformed and springs back. The stiffer the material, the faster sound travels through it.
- electric field
- The region of influence around a charged object, which exerts a force on other charges.
- electromagnetic spectrum
- The full range of electromagnetic waves, from radio waves through visible light to gamma rays.
- electromagnetic wave
- A transverse wave of oscillating electric and magnetic fields that regenerate each other, able to cross a vacuum.
- epicentre
- The point on Earth's surface directly above where an earthquake originates.
- equilibrium
- The undisturbed rest position of the medium, about which particles oscillate as a wave passes.
F
- Faraday's law
- A changing magnetic field creates an electric field in the surrounding space.
- fiber optics
- Transmitting data as pulses of light along thin glass fibres by total internal reflection.
- frequency
- The number of complete wave cycles passing a fixed point each second, measured in hertz (Hz).
- Fresnel lens
- A thin lens of concentric prismatic rings, invented in 1821 and first used to extend the reach of lighthouse beams.
- fundamental
- The lowest frequency at which a fixed string or air column naturally vibrates.
G
- gamma ray
- The highest-energy electromagnetic radiation, with wavelengths below 0.01 nm, used in cancer treatment.
- general relativity
- Einstein's 1915 theory describing gravity as the curvature of spacetime.
- gravitational wave
- A transverse ripple in spacetime itself, radiated by violent events such as two black holes merging.
H
- harmonic
- A vibration at a whole-number multiple of the fundamental frequency, also called an overtone.
- hertz
- The unit of frequency: one hertz (Hz) equals one cycle per second.
- Hooke's law
- The restoring force on a displaced object is proportional to how far it was displaced: F = −kx.
- Huygens' Principle
- The rule that every point on a wavefront acts as a source of new spherical wavelets.
I
- in phase
- Two waves are in phase when their crests and troughs line up, so they reinforce each other.
- inertia
- A material's resistance to being accelerated. Greater inertia slows a wave down.
- infrared
- Electromagnetic radiation just beyond red light (700 nm–1 mm), felt as heat and used in thermal cameras.
- infrasound
- Sound below about 20 Hz, too low for humans to hear. Elephants, whales and earthquakes all produce it.
- inner core
- Earth's solid nickel-iron centre, about 1,220 km in radius, discovered by Inge Lehmann in 1936.
- interference
- The result of two waves overlapping — reinforcing each other in some places and cancelling in others.
- isothermal
- A process slow enough that the temperature stays constant as heat flows away. Newton wrongly assumed sound behaved this way.
K
- kinetic energy
- The energy of motion, greatest as an oscillating particle passes through its rest position at top speed.
L
- LCD
- A liquid crystal display, which forms an image by rotating the polarization of light between two polarizing filters.
- Lehmann Discontinuity
- The boundary between Earth's liquid outer core and its solid inner core, named after Inge Lehmann.
- light-year
- The distance light travels in one year — about 9.5 trillion kilometres.
- LIGO
- The Laser Interferometer Gravitational-Wave Observatory, which made the first direct detection of gravitational waves in 2015.
- linear mass density
- The mass per unit length of a string, symbol μ, in kg/m. Heavier strings carry slower waves.
- liquid crystal
- A material whose molecules rotate under an electric field, used with polarizers to control each pixel of an LCD.
- longitudinal wave
- A wave whose particles oscillate back and forth along the same direction the wave travels. Sound is the classic example.
- luminiferous aether
- The invisible medium that physicists once wrongly believed light needed in order to travel.
M
- magnetic field
- The region of influence around a magnet or current, which exerts a force on moving charges.
- Malus's Law
- The rule I = I₀cos²θ, giving how much polarized light passes a second polarizer set at angle θ.
- mantle
- The thick layer of solid rock between Earth's crust and outer core, about 35 to 2,900 km deep.
- Maxwell's equations
- The four equations unifying electricity and magnetism, which predicted electromagnetic waves travelling at the speed of light.
- medium
- The material a mechanical wave travels through, such as rope, water or rock. Electromagnetic waves need no medium at all.
- microwave
- Electromagnetic radiation from 1 mm to 1 m, used for cooking, radar and 5G networks.
- momentum
- The product of mass and velocity — greatest for an oscillating particle as it races through equilibrium.
N
- nanometre
- One billionth of a metre (10⁻⁹ m) — the usual unit for measuring the wavelength of light.
- node
- A point on a standing wave that never moves, where the two overlapping waves always cancel.
- noise-cancelling
- Technology that plays a sound wave exactly out of phase with ambient noise, so the two cancel in your ear.
- non-destructive testing
- Inspecting the inside of a component with waves instead of cutting it open — used on bridges, pipelines and aircraft.
O
- oscillation
- A repeated back-and-forth motion about a central rest position.
- out of phase
- Two waves are out of phase when one's crest meets the other's trough, so they cancel each other out.
- outer core
- Earth's liquid nickel-iron layer, roughly 2,900 to 5,150 km deep. Being liquid, it blocks S-waves entirely.
- overtone
- Any vibration above the fundamental frequency, at a whole-number multiple of it.
P
- P-wave
- A primary, longitudinal seismic wave arriving first at 6–13 km/s, able to travel through both solids and liquids.
- period
- The time taken for one complete wave cycle to pass a point. It is the reciprocal of frequency: T = 1/f.
- perpendicular
- At a right angle (90°) to something. In a transverse wave the medium oscillates perpendicular to the direction the wave travels.
- phase
- How far through its cycle an oscillation has progressed at a given moment.
- photoelectric effect
- Einstein's 1905 finding that light knocks electrons from metal in discrete packets, showing light also acts as particles.
- photon
- A single quantum, or particle, of light. Its energy is E = hf, so higher-frequency light carries more energy per photon.
- Planck's constant
- The fundamental constant h = 6.626 × 10⁻³⁴ J·s, which links a photon's energy to its frequency.
- Poisson's spot
- The bright point at the centre of a circular shadow — predicted as an absurdity, then observed, confirming light is a wave.
- polarization
- Restricting a transverse wave's oscillation to a single plane. Only transverse waves can be polarized — sound cannot.
- polarized
- Describes a transverse wave whose oscillation has been restricted to one single plane.
- polarizer
- A filter that transmits only the component of a transverse wave oscillating in one particular plane.
- potential energy
- Stored energy, greatest when an oscillating particle sits at maximum displacement at a crest or trough.
- presbycusis
- The natural, age-related loss of high-frequency hearing as the hair cells of the inner ear lose sensitivity.
- pressure wave
- A wave carried by variations in the pressure of a medium — another name for a longitudinal wave such as sound.
- propagation
- The travelling of a wave through space or a medium, carrying energy forward without carrying matter with it.
Q
- quantum cryptography
- Encryption that encodes information in the quantum states of individual photons, making eavesdropping detectable.
R
- radio wave
- The longest electromagnetic waves, from 1 mm up to 100 km, used for broadcasting, Wi-Fi and communication.
- rarefaction
- A region of a longitudinal wave where the particles of the medium are spread further apart.
- refraction
- The bending of a wave as it changes speed on entering a new medium.
- restoring force
- The force pulling a displaced particle back toward its rest position — the thing that keeps an oscillation going.
S
- S-wave
- A secondary, transverse seismic wave that shears rock sideways at 3–6 km/s. It cannot travel through liquid.
- seismic wave
- A wave of energy travelling through the Earth, generated by earthquakes and used to map the planet's interior.
- shadow zone
- The region beyond 103° from an earthquake's epicentre where S-waves never arrive — proof that Earth's outer core is liquid.
- shear stress
- A force acting to slide one layer of a material sideways past another. Solids resist it; liquids and gases simply flow.
- shear wave
- Another name for a transverse mechanical wave, which distorts a material sideways rather than compressing it.
- solar mass
- The mass of the Sun, used as a standard unit for weighing stars and black holes.
- sonar
- Locating objects underwater by emitting a sound pulse and timing the returning echo.
- spacetime
- The combined four-dimensional fabric of space and time that gravity bends and gravitational waves ripple through.
- speed of light
- 299,792,458 metres per second in a vacuum — the speed of every electromagnetic wave, and the universe's speed limit.
- standing wave
- A wave pattern that oscillates in place rather than travelling, formed when a wave reflects and overlaps itself.
- superposition
- When two waves overlap, their displacements simply add together at every point.
T
- tension
- The pulling force stretching a string, measured in newtons. More tension means faster waves and a higher pitch.
- timbre
- The tonal 'colour' of a sound, set by the particular mix of harmonics it contains.
- total internal reflection
- When light strikes a boundary at a shallow enough angle that it reflects entirely back instead of escaping.
- transverse wave
- A wave whose oscillation is at right angles to its direction of travel. Light, guitar strings and seismic S-waves are all transverse.
- trough
- The lowest point of a transverse wave, where displacement below the rest position is greatest.
U
- ultrasonic
- Sound or vibration above the range of human hearing, used to inspect solid objects for hidden flaws.
- ultrasound
- Sound above about 20,000 Hz, beyond human hearing. Used by bats and dolphins, and in medical imaging at 1 to 20 MHz.
- ultraviolet
- Electromagnetic radiation just beyond violet light (10–400 nm), used for sterilisation and responsible for sunburn.
V
- vacuum
- Space containing no matter. Light crosses it freely; sound cannot travel through it at all.
- visible light
- The narrow band of electromagnetic radiation the human eye can see, roughly 400 nm (violet) to 700 nm (red).
W
- wave speed
- How fast the wave pattern travels through the medium, given by the universal wave equation v = fλ.
- wave-particle duality
- The principle that light and matter behave as both waves and particles, depending on how they are observed.
- wavefront
- A surface joining all points of a wave that are in the same phase, such as a whole line of crests.
- wavelength
- The distance between two identical consecutive points on a wave, such as crest to crest. Symbol λ, measured in metres.
X
- X-ray
- High-energy electromagnetic radiation (0.01–10 nm) that passes through soft tissue but is absorbed by bone.