Physics terms in Wave behaviour

A continuous spectrum crossed by dark lines, produced when light passes through a cooler gas that absorbs photons whose energies match its energy-level differences.

The maximum displacement from the equilibrium position, of an oscillating body or of the particles of a medium carrying a wave; symbol $x_0$.

The angle between an incident ray and the normal at the point where the ray meets a boundary.

$2\pi$ times the frequency of an oscillation, $\omega =2\pi f=2\pi \mathrm{/}T$, measured in ${\text{rad s}}^{-1}$.

A point on a standing wave where the amplitude of oscillation is a maximum, midway between two adjacent nodes.

Two oscillations of the same frequency are in antiphase when their phase difference is $\pi$ rad (half a cycle), so one is at its positive maximum when the other is at its negative maximum.

An opening, such as a gap or slit, through which a wave passes.

A decrease in the observed wavelength (and increase in frequency) of electromagnetic waves from a source moving towards the observer.

The condition a standing wave must meet at the end of its medium: a node at a fixed or closed end, an antinode at a free or open end.

The bright region at the centre of a single-slit diffraction pattern, twice as wide as each side maximum and much brighter.

A pipe closed at one end and open at the other; the closed end is a displacement node, the first harmonic has $L=\lambda \mathrm{/}4$ and only odd harmonics are present.

Describes waves or sources with the same frequency and a constant phase difference.

A region of a longitudinal wave where the particles of the medium are closer together than normal, so the pressure is higher.

Superposition of waves that meet in phase, giving a resultant amplitude larger than either wave's; for in-phase sources it occurs where the path difference is $n\lambda$.

A point of maximum positive displacement on a transverse wave.

The angle of incidence, in the medium where a wave is slower, for which the angle of refraction is 90°. For light, $\sin {\theta }_{\mathrm{c}}=n_2\mathrm{/}{n}_1$.

The amount of damping at which a displaced system returns to equilibrium in the shortest time without oscillating.

One complete oscillation, ending when the body is next at the same position moving in the same direction.

The loss of energy from an oscillating system to its surroundings because of resistive forces, which makes the amplitude decrease.

Superposition of waves that meet in antiphase, so that their displacements cancel; for in-phase sources it occurs where the path difference is $(n+\frac{1}{2})\lambda$.

The spreading of a wave as it passes through an aperture or past the edge of an obstacle; greatest when the aperture is about the same size as the wavelength.

A large number of identical, equally spaced parallel slits, which produces very sharp maxima at angles given by $n\lambda =d\sin \theta$.

The change in the observed frequency and wavelength of a wave when the source and the observer move relative to each other.

A medical technique that measures the speed and direction of blood flow from the frequency shift of ultrasound reflected by moving blood cells.

The frequency of the periodic external force acting on an oscillating system.

The continuous range of electromagnetic waves ordered by wavelength: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves.

A transverse wave of electric and magnetic fields oscillating in phase, perpendicular to each other and to the direction of travel; it travels through a vacuum at $c=3.00\times {10}^8{\text{m s}}^{-1}$.

The position where the resultant force on an oscillating body is zero; displacement in an oscillation or a wave is measured from it.

A planet orbiting a star other than the Sun; many are found from the periodic Doppler shift of their star's spectral lines.

The standing wave of lowest frequency that a system can support.

The first dark direction either side of the centre of a single-slit pattern, at $\theta =\lambda \mathrm{/}b$ (radians, small angles), where $b$ is the slit width.

An end of a string that cannot move, such as a clamp; it is always a node of a standing wave.

An oscillation maintained by a periodic external force; in the steady state the system oscillates at the driving frequency.

An end of a string that can move freely, such as a light ring sliding on a smooth rod; it is always an antinode of a standing wave.

The number of oscillations or cycles per unit time; symbol $f$, measured in hertz (Hz), with $f=1\mathrm{/}T$.

One of the bright or dark bands in an interference or diffraction pattern.

The distance between the centres of neighbouring bright (or dark) fringes; for a double slit, $s=\lambda D\mathrm{/}d$.

High-energy photons emitted when a nucleus falls from an excited state to a lower energy state; the numbers of protons and neutrons do not change.

The distance $d$ between neighbouring slits of a diffraction grating: $d=1\mathrm{/}N$ for $N$ lines per metre.

One of the standing waves a system can support; the nth harmonic has a frequency n times that of the first harmonic.

Damping greater than critical: the system does not oscillate and returns to equilibrium more slowly than with critical damping.

Two oscillations of the same frequency are in phase when their phase difference is zero, so they reach their maxima at the same time.

The power carried by a wave per unit area perpendicular to its direction of travel, in ${\text{W m}}^{-2}$; for a given frequency it is proportional to the square of the amplitude.

The superposition of waves from coherent sources, giving a steady pattern of maxima and minima.

Describes an oscillation whose period does not depend on its amplitude, as in simple harmonic motion.

The angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in the same plane.

Damping small enough that the system still oscillates, with an amplitude that decreases gradually.

The straight line joining an observer and the body observed; only motion along it produces a Doppler shift.

A wave in which the particles of the medium oscillate parallel to the direction of energy transfer.

The part of a standing wave between two adjacent nodes; it is half a wavelength long and all its points oscillate in phase.

A mass attached to a spring that oscillates with simple harmonic motion of period $T=2\pi \sqrt{m\mathrm{/}k}$.

A wave that needs a material medium whose particles oscillate to pass on the disturbance, such as sound, water waves or waves on a string.

The material through which a mechanical wave travels, whose particles oscillate as the wave passes.

A double-slit or grating maximum that does not appear because it falls on a minimum of the single-slit envelope; for a double slit it happens when $d\mathrm{/}b$ equals the order $n$.

Describes light (or another wave) of a single wavelength and frequency.

A point on a standing wave where the displacement is always zero. Adjacent nodes are half a wavelength apart.

A line drawn perpendicular to a surface or boundary at the point where a ray meets it. Angles of incidence, reflection and refraction are measured from it.

A pipe open at both ends; each end is a displacement antinode, the first harmonic has $L=\lambda \mathrm{/}2$ and all harmonics are present.

A thin strand of glass with a core surrounded by cladding of lower refractive index, which guides light along it by repeated total internal reflection.

Describes the medium with the higher refractive index of two, in which light travels more slowly.

The whole number $n$ of wavelengths in the path difference between light from neighbouring slits at a grating or double-slit maximum.

A repeated back-and-forth motion of a body about a fixed equilibrium position.

A longitudinal seismic wave; it travels faster than S waves and through solids and liquids.

The difference between the distances travelled by two waves from their sources to a given point.

The stage an oscillation has reached in its cycle, expressed as an angle in radians, $\omega t+\varphi$.

The angle $\varphi$, in radians, that fixes the position of a simple harmonic oscillator in its cycle at $t=0$.

The difference in phase between two oscillations or two points on a wave of the same frequency, in radians: $\mathrm{\Delta }\varphi =2\pi \mathrm{\Delta }t\mathrm{/}T$.

How high or low a sound is heard to be; a higher frequency is heard as a higher pitch.

When two or more waves meet at a point, the resultant displacement is the vector sum of the displacements that each wave would produce on its own.

A single short disturbance that travels through a medium, as opposed to a continuous wave.

A system that sends out microwaves or radio waves and detects their reflections; the Doppler shift of the reflected waves, $\mathrm{\Delta }f\mathrm{/}f\approx 2v\mathrm{/}c$, gives the speed of the target.

The component of a body's velocity along the line of sight, towards or away from the observer; it is the part of the motion that a Doppler shift measures.

A region of a longitudinal wave where the particles of the medium are further apart than normal, so the pressure is lower.

A line showing the direction in which a wave travels and transfers energy; it is always perpendicular to the wavefronts.

An increase in the observed wavelength (and decrease in frequency) of electromagnetic waves from a source moving away from the observer.

A circle of radius $x_0$ round which a point moves at angular speed $\omega$; the projection of the point onto a diameter moves in simple harmonic motion, and the angle of its radius is the phase $\omega t+\varphi$.

The change in direction of a wave at a boundary so that it returns into the medium it came from, with its speed, frequency and wavelength unchanged.

The change in direction of a wave as it passes from one medium into another in which its speed is different.

The ratio of the speed of light in a vacuum to the speed of light in a medium, $n=c\mathrm{/}v$. It has no unit and is at least 1.

The velocity of one body as measured by an observer moving with another body; for light, only the relative velocity of source and observer along the line joining them sets the Doppler shift.

A graph of the amplitude of a forced oscillation against the driving frequency, with a peak near the natural frequency.

A force that acts on a displaced body in the direction of its equilibrium position.

A transverse seismic wave; it travels more slowly than P waves and cannot pass through liquids.

A wave that carries energy through the Earth from an earthquake or explosion.

The cone-shaped pile-up of wavefronts produced by a source moving faster than the waves it emits, heard for sound as a sonic boom.

Oscillation in which the acceleration is proportional to the displacement from a fixed equilibrium position and always directed towards it: $a=-{\omega }^{2}x$.

A small dense bob on a light inextensible string that, for small angles, oscillates with simple harmonic motion of period $T=2\pi \sqrt{l\mathrm{/}g}$.

For an angle $\theta$ in radians that is small (below about 0.2 rad), $\sin \theta \approx \tan \theta \approx \theta$.

For a wave refracted at a boundary, $n_1\sin {\theta }_1=n_2\sin {\theta }_2$, or equivalently $\sin {\theta }_1\mathrm{/}\sin {\theta }_2=v_1\mathrm{/}{v}_2$, with angles measured from the normal.

A longitudinal mechanical wave of compressions and rarefactions; it needs a medium and cannot travel through a vacuum.

A narrow bright or dark line in a spectrum at a wavelength emitted or absorbed by a particular element; its shift from the laboratory wavelength measures the radial velocity of the source.

The pattern formed when two waves of the same frequency, wavelength and amplitude travelling in opposite directions superpose; it oscillates in place and transfers no net energy.

The positive feedback in which pedestrians on a swaying footbridge fall into step with the sway, so their sideways foot forces increase it.

The complete reflection of a wave at a boundary with a medium in which it would travel faster, when the angle of incidence exceeds the critical angle.

The passage of a wave across a boundary from one medium into another. The frequency is unchanged; the speed and wavelength change in the same ratio.

A wave in which the oscillations are perpendicular to the direction of energy transfer.

A wave whose disturbance moves through a medium or through space, transferring energy without any net transfer of matter.

A point of maximum negative displacement on a transverse wave.

A mass on a spring (with damping) attached to a structure and tuned to the structure's natural frequency, so that it oscillates out of phase with the structure and absorbs energy from it.

Sound with a frequency above the upper limit of human hearing, about 20 kHz.

A disturbance that transfers energy from one place to another without a net transfer of matter.

The speed at which a wave transfers energy through a medium or through space, $v=f\lambda$.

A line or surface joining neighbouring points of a wave that are vibrating in phase, such as all the points on one crest. Neighbouring wavefronts are usually drawn one wavelength apart.

The shortest distance between two points on a wave that are in phase, such as adjacent crests or adjacent compressions; symbol $\lambda$, in metres.