What is a wave in physics?

In physics, a wave is the organized and regular propagation of disturbances from one place to another. Waves can be observed on water surfaces, but sound, light, and subatomic particle motion also display wave characteristics. The simplest waves oscillate periodically with a fixed frequency and wavelength. Mechanical waves, like sound, need a medium to travel through. Electromagnetic waves , such as light, can travel through a vacuum without needing a medium. The properties of the medium affect how a wave propagates.

What are the main types of waves?

Waves transfer disturbances from one point to another in a regular manner. The simplest waves oscillate periodically with fixed frequency and wavelength. There are two main types of waves: * Longitudinal Waves These waves, like sound, consist of alternating compressions and rarefactions within a medium. The medium vibrates along the path of propagation. * Transverse Waves These waves are like those on water, with the surface going up and down. The disturbance occurs in a direction perpendicular to that in which the wave is propagating. Seismic waves have three main types: P (primary), S (secondary), and surface waves (Love and Rayleigh waves). Electromagnetic waves, such as radio waves , visible light , and gamma rays , do not require a medium and can travel through a vacuum.

How do transverse waves differ from longitudinal waves?

Transverse waves and longitudinal waves are the two main types of wave motion. In transverse waves, particles oscillate at right angles to the wave's direction, like ripples on water or light waves. Conversely, in longitudinal waves, particles oscillate in the same direction as the wave's advance, creating compressions and rarefactions, as seen in sound waves. Transverse waves have crests (high points) and troughs (low points), while longitudinal waves have compressions and rarefactions analogous to crests and troughs.

What are the basic properties of waves, like wavelength and amplitude?

Waves involve the propagation of disturbances through a medium or space. Key properties include wavelength and amplitude. Wavelength : This is the distance between corresponding points on two consecutive waves, such as crest to crest or trough to trough. It is commonly represented by the Greek letter lambda ((\lambda)). Amplitude : Amplitude refers to the maximum displacement of a point on a wave from its equilibrium position. For a transverse wave, like a wave on a string, it's the maximum displacement of any point on the string when at rest. For a longitudinal wave, such as a sound wave, it's the maximum displacement of a particle from its equilibrium.

How do sound waves travel through different materials?

Sound waves are disturbances that transfer energy through a medium via particle vibration. These vibrations create compressions and rarefactions, areas of high and low pressure, which propagate as the sound wave. Sound waves cannot travel through a vacuum because they require a medium to facilitate disturbances. The speed of sound varies depending on the medium and its properties. Sound travels faster in liquids and solids than in gases because the particles are more closely packed. The speed of sound in dry air at 0 °C is approximately 331.29 meters per second, while in liquid water at 8 °C, it's about 1,439 meters per second. The ability of a sound source to radiate sound is affected by the acoustic impedance of the medium. Reflection occurs when a sound wave encounters a mismatch between two mediums.

How do light waves behave when they pass through water or glass?

When light waves pass from one transparent medium to another, such as from air into water or glass, they undergo refraction , changing direction due to a change in speed. Light travels more slowly in a denser medium. The angle of incidence is the angle between an incoming wave and a line perpendicular to the surface. As light enters a different medium, it bends, and this change in direction is described by Snell's law : (n_1 \sin \theta_1 = n_2 \sin \theta_2), where (n) represents the refractive index of the material, and (\theta) is the angle of incidence and refraction. The refractive index of a material is the ratio of the speed of light in a vacuum to its speed in the medium. Different wavelengths of light refract differently; for example, blue light has a higher refractive index than red light.

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$How does refraction affect the direction of a wave?$

$What is diffraction?$

Three graphs illustrate types of light wave interference. The top shows constructive interference with two waves combining to form a larger wave. The middle shows partial destructive interference, where the waves partially cancel, resulting in a smaller combined wave. The bottom shows complete destructive interference, where two waves of equal amplitude and opposite phase cancel each other out, resulting in a flat line.

Diagram illustrating the Doppler shift in astronomy, showing how absorption lines in a color spectrum move toward violet for approaching objects and toward red for receding objects, with labeled comparison spectra.

Figure 18: Progression of elliptically polarized wave (see text).

A simple diagram showing two blue waves: the wave on the left has a shorter wavelength with closely spaced peaks, while the wave on the right has a longer wavelength with peaks spaced farther apart.

View an animation explaining how small things like electrons and protons can behave like particles and sometimes like waves

Understand the uncertainty principle as defined by the properties of waves

Understanding Heisenberg's uncertainty principle in subatomic particles

At a Glance

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Quick Facts

Key People:: Hermann von Helmholtz; Lord Rayleigh; Kip Thorne; Ernst Mach

Related Topics:: radiation; electromagnetic radiation; sound; wave; Gravitational wave

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Sep. 21, 2026 •History

Top Questions

What is a wave in physics?
In physics, a wave is the organized and regular propagation of disturbances from one place to another. Waves can be observed on water surfaces, but sound, light, and subatomic particle motion also display wave characteristics.

The simplest waves oscillate periodically with a fixed frequency and wavelength. Mechanical waves, like sound, need a medium to travel through. Electromagnetic waves, such as light, can travel through a vacuum without needing a medium. The properties of the medium affect how a wave propagates.
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What are the main types of waves?
Waves transfer disturbances from one point to another in a regular manner. The simplest waves oscillate periodically with fixed frequency and wavelength.

There are two main types of waves:
* Longitudinal Waves These waves, like sound, consist of alternating compressions and rarefactions within a medium. The medium vibrates along the path of propagation.
* Transverse Waves These waves are like those on water, with the surface going up and down. The disturbance occurs in a direction perpendicular to that in which the wave is propagating.

Seismic waves have three main types: P (primary), S (secondary), and surface waves (Love and Rayleigh waves). Electromagnetic waves, such as radio waves, visible light, and gamma rays, do not require a medium and can travel through a vacuum.
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How do transverse waves differ from longitudinal waves?
Transverse waves and longitudinal waves are the two main types of wave motion. In transverse waves, particles oscillate at right angles to the wave's direction, like ripples on water or light waves. Conversely, in longitudinal waves, particles oscillate in the same direction as the wave's advance, creating compressions and rarefactions, as seen in sound waves. Transverse waves have crests (high points) and troughs (low points), while longitudinal waves have compressions and rarefactions analogous to crests and troughs.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are the basic properties of waves, like wavelength and amplitude?
Waves involve the propagation of disturbances through a medium or space. Key properties include wavelength and amplitude.
- Wavelength: This is the distance between corresponding points on two consecutive waves, such as crest to crest or trough to trough. It is commonly represented by the Greek letter lambda ((\lambda)).
- Amplitude: Amplitude refers to the maximum displacement of a point on a wave from its equilibrium position. For a transverse wave, like a wave on a string, it's the maximum displacement of any point on the string when at rest. For a longitudinal wave, such as a sound wave, it's the maximum displacement of a particle from its equilibrium.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do sound waves travel through different materials?
Sound waves are disturbances that transfer energy through a medium via particle vibration. These vibrations create compressions and rarefactions, areas of high and low pressure, which propagate as the sound wave.

Sound waves cannot travel through a vacuum because they require a medium to facilitate disturbances. The speed of sound varies depending on the medium and its properties. Sound travels faster in liquids and solids than in gases because the particles are more closely packed.

The speed of sound in dry air at 0 °C is approximately 331.29 meters per second, while in liquid water at 8 °C, it's about 1,439 meters per second. The ability of a sound source to radiate sound is affected by the acoustic impedance of the medium. Reflection occurs when a sound wave encounters a mismatch between two mediums.
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How do light waves behave when they pass through water or glass?
When light waves pass from one transparent medium to another, such as from air into water or glass, they undergo refraction, changing direction due to a change in speed. Light travels more slowly in a denser medium.

The angle of incidence is the angle between an incoming wave and a line perpendicular to the surface. As light enters a different medium, it bends, and this change in direction is described by Snell's law: (n_1 \sin \theta_1 = n_2 \sin \theta_2), where (n) represents the refractive index of the material, and (\theta) is the angle of incidence and refraction.

The refractive index of a material is the ratio of the speed of light in a vacuum to its speed in the medium. Different wavelengths of light refract differently; for example, blue light has a higher refractive index than red light.
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wave, propagation of disturbances from place to place in a regular and organized way. Most familiar are surface waves that travel on water, but sound, light, and the motion of subatomic particles all exhibit wavelike properties. In the simplest waves, the disturbance oscillates periodically (see periodic motion) with a fixed frequency and wavelength. Mechanical waves, such as sound, require a medium through which to travel, while electromagnetic waves (see electromagnetic radiation) do not require a medium and can be propagated through a vacuum. Propagation of a wave through a medium depends on the medium’s properties. See also seismic wave.

Types and features of waves

Waves come in two kinds, longitudinal and transverse. Transverse waves are like those on water, with the surface going up and down, and longitudinal waves are like of those of sound, consisting of alternating compressions and rarefactions in a medium. The high point of a transverse wave is a called the crest, and the low point is called the trough. For longitudinal waves, the compressions and rarefactions are analogous to the crests and troughs of transverse waves. The distance between successive crests or troughs is called the wavelength. The height of a wave is the amplitude. How many crests or troughs pass a specific point during a unit of time is called the frequency. The velocity of a wave can be expressed as the wavelength multiplied by the frequency.

Waves can travel immense distances even though the oscillation at one point is very small. For example, a thunderclap can be heard kilometres away, yet the sound carried manifests itself at any point only as minute compressions and rarefactions of the air.

Italian-born physicist Dr. Enrico Fermi draws a diagram at a blackboard with mathematical equations. circa 1950.

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Physics and Natural Law

Wave behaviour

Waves display several basic phenomena. In reflection, a wave encounters an obstacle and is reflected back. In refraction, a wave bends when it enters a medium through which it has a different speed. In diffraction, waves bend when they pass around small obstacles and spread out when they pass through small openings. In interference, when two waves meet, they can interfere constructively, creating a wave with larger amplitude than the original waves, or destructively, creating a wave with a smaller (or even zero) amplitude.

Reflection

When waves hit a boundary and are reflected, the angle of incidence equals the angle of reflection. The angle of incidence is the angle between the direction of motion of the wave and a line drawn perpendicular to the reflecting boundary.

Refraction

The speed of a wave depends on the properties of the medium through which it travels. For example, sound travels much faster through water than through air. When a wave enters at an angle a medium through which its speed would be slower, the wave is bent toward the perpendicular. When a wave enters at an angle a medium in which its speed would be increased, the opposite effect happens. With light, this change can be expressed by using Snell’s law of refraction.

Diffraction

When a wave encounters a small obstacle or a small opening (that is, small compared with the wavelength of the wave), the wave can bend around the obstacle or pass through the opening and then spread out. This bending or spreading out is called diffraction.

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Interference

The waves from two or more centres of disturbance may reinforce each other in some directions and cancel in others. This phenomenon is called the interference of waves. It is easy to see how this may happen. Consider two sources producing waves of the same wavelength and in phase; that is, at their origin the crests of the waves occur at the same time. If a point P is equidistant from both sources, the crests arrive at P simultaneously and reinforce each other. Similarly, the troughs arrive simultaneously and become deeper. The same situation occurs if the distances to point P are unequal but differ by one or more full wavelengths. If, however, the distances differ by half a wavelength or by an odd number of half wavelengths, then the crests of one wave will coincide with the troughs of the other and the intensity of the resultant wave is decreased. When two such waves are of equal intensity, they will cancel each other completely. Intermediate situations arise in those directions in which the distances traveled by the two waves differ by some other fraction of a wavelength, the waves tending either to reinforce or to cancel each other.

Doppler effect

When the source of a wave moves relative to an observer, the observer notices a change in the frequency of the wave. This change is called the Doppler effect, after its discoverer, Austrian physicist Christian Doppler.

Consider a source emitting a wave such as light or sound of frequency ν moving away from an observer at velocity v. The successive crests of the light waves will reach the observer at longer intervals than if the observer were at rest, and calculation shows that the observer will receive them with a frequency ν(1−v/c), where c is the velocity of the wave. The frequency of the wave will appear to the observer slightly lower than it would if the source were at rest. If the source is approaching, the frequency will be higher.

In sound this effect is an everyday experience; when a blowing horn is passed on the highway, the observer may notice that the pitch of the note seems to change. The Doppler effect for light waves is evident in spectroscopy. A shift to higher frequencies is called a blueshift, and a shift to lower frequencies is called a redshift. The redshifted light from other galaxies is evidence of the expansion of the universe.

Standing waves

If a wave is confined to a closed space, it undergoes both reflection and interference. For example, consider a tube of length l. A disturbance anywhere in the air in the tube will be reflected from both ends and produce in general a series of waves traveling in both directions along the tube. From the geometry of the situation and the finite constant value of acoustic velocity, these must be periodic waves with frequencies fixed by the boundary conditions at the end of the tube. The allowed frequencies of the waves in the tube satisfy sin kl = 0; i.e., the allowed frequencies are ν = nv/2l, where n is any integer and v is the acoustic velocity in the tube. These are the frequencies of harmonic waves that can exist in the tube and still satisfy the boundary conditions at the ends. They are called the characteristic frequencies or normal modes of vibration of the air column. The fundamental frequency (n = 1) is ν = v/2l.

The higher frequencies, called harmonics or overtones, are multiples of the fundamental. It is customary to refer to the fundamental as the first harmonic; n = 2 gives the second harmonic or first overtone, and so on. Approximately the same set of characteristic frequencies hold for a cylindrical tube open at both ends, though the boundary conditions are different.

There are positions in the tube at which the displacement of the air is zero at all times. This cannot take place in a progressive wave; thus, the wave disturbance corresponding to a normal mode is known as a standing wave. The positions of continuous zero displacement are known as nodes, while the positions for which there is maximum displacement are called antinodes. The distance between successive nodes is equal to a half wavelength of the particular mode.

This article was most recently revised and updated by Erik Gregersen.

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Types and features of waves

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Wave behaviour

Reflection

Refraction

Diffraction

Interference

Doppler effect

Standing waves