Glossary — Waves

Antinode — a point on a stationary wave where the amplitude is largest.

Coherence — a property of two sources that have the same frequency and a constant phase difference, so their interference pattern stays still.

Compression — a place in a longitudinal wave where the particles are closer together than usual.

Constructive interference — the meeting of two waves in phase, so their displacements add and the resultant amplitude is larger.

Destructive interference — the meeting of two waves antiphase, so their displacements subtract and the resultant amplitude is smaller.

Diffraction — the spreading of a wave when it passes through a gap or past an edge.

Diffraction grating — a slide with many parallel slits, close together, used to split light into its wavelengths. Its maxima are at $d\sin\theta = n\lambda$.

Fringe spacing — the distance from one bright fringe to the next in a two-slit pattern, $w = \dfrac{\lambda D}{s}$.

Fundamental frequency — the lowest frequency at which a string or an air column can form a stationary wave. It is also called the first harmonic.

Harmonic — one of the frequencies at which a string or an air column forms a stationary wave. The $n$th harmonic has $n$ half wavelengths on the string or $n$ times the fundamental frequency.

Longitudinal wave — a wave in which the particles vibrate along the direction in which the wave travels. Sound is a longitudinal wave.

Node — a point on a stationary wave where the displacement is always zero.

Order (of a maximum) — the number $n$ in the grating equation $d\sin\theta = n\lambda$. Order 0 is the straight-through direction, order 1 is the first maximum on each side, and so on.

Path difference — the difference in the distances travelled by two waves from their sources to the same point. It decides whether the waves reinforce or cancel.

Phase — the stage a wave has reached in its cycle, measured as an angle.

Phase difference — how far apart in the cycle two points on a wave are. One wavelength is $360^\circ$ or $2\pi$ radians.

Plane polarised — describes light whose electric field vibrates in one plane only.

Polarisation — the restricting of a transverse wave's vibration to one plane. Only transverse waves can be polarised.

Polarising filter — a filter that transmits only the component of a transverse wave's vibration along its axis.

Progressive wave — a wave that carries energy from one place to another without carrying matter with it.

Rarefaction — a place in a longitudinal wave where the particles are further apart than usual.

Stationary wave — a wave formed when two waves of the same frequency travel in opposite directions and superpose. It has nodes and antinodes and transfers no net energy.

Superposition, principle of — when two waves meet, the resultant displacement at each point is the sum of the displacements the waves would have on their own.

Transverse wave — a wave in which the particles vibrate at right angles to the direction in which the wave travels. Light is a transverse wave.

Wavelength — the distance between two neighbouring points on a wave that are in step, such as two crests.