What Is a Wave?

In physics, a wave is a disturbance that travels through space or matter, carrying energy from one place to another without permanently moving the medium. Think of a ripple spreading on a pond: the water moves up and down, but it does not flow outward. Only the energy moves. Waves are everywhere: water ripples, sound, light, earthquakes, radio signals and even the vibrations in a guitar string. Understanding waves is one of the most important parts of physics, because almost every signal in the universe (including the ones you see and hear right now) is a wave.

  • What it isA travelling disturbanceCarries energy, not matter
  • Two main typesTransverse and longitudinalPlus surface waves
  • Speed of sound343 m/s in air1,500 m/s in water
  • Speed of light300,000 km/sIn a vacuum
  • WavelengthCrest-to-crest distanceCan be tiny or huge
  • FrequencyWaves per secondMeasured in hertz (Hz)

Parts of a wave

  • Crest: the top of a wave.
  • Trough: the bottom of a wave.
  • Wavelength: the distance from one crest to the next.
  • Amplitude: how high the crest rises above the middle (the strength of the wave).
  • Frequency: how many full waves pass a fixed point each second.
  • Period: the time for one full wave to pass.
  • Speed: how fast the wave travels through its medium.

The relationship is simple: speed = wavelength x frequency. A wave with a long wavelength and a low frequency travels at the same speed as one with a short wavelength and a high frequency, if both are in the same medium.

Types of waves

Waves are usually grouped by how they move:

  • Transverse waves: particles move at right angles to the direction the wave travels. Examples: water ripples, waves on a string, light, electromagnetic waves.
  • Longitudinal waves: particles move back and forth along the same direction as the wave travels. Examples: sound, "slinky" waves, earthquake P-waves.
  • Surface waves: a mix of both, like ocean waves where water moves in small circles.
Fact Water in an ocean wave does not actually travel across the sea: it moves in small circles in place. When a wave reaches the shore, what looks like water rushing forward is actually energy unloading from the deep water below, briefly throwing the surface water forward as the wave breaks.

Waves around you

  • Sound waves: air pressure ripples, made by anything that vibrates. (See what is sound.)
  • Water waves: visible disturbances on the surface of seas, lakes and puddles.
  • Light waves: electromagnetic waves you can see, with wavelengths between about 400 and 700 nanometres.
  • Radio waves: electromagnetic waves with much longer wavelengths, used for broadcasting, mobile phones and Wi-Fi.
  • X-rays and microwaves: electromagnetic waves with different wavelengths, used in medicine and cooking.
  • Earthquake waves: travel through Earths interior at thousands of km/h, both as P-waves (longitudinal) and S-waves (transverse).
  • Vibrations: in strings, drums, metal bars, even bridges and buildings.
  • Gravitational waves: ripples in space-time itself, first detected in 2015.

What waves can do

  • Reflect: bounce off surfaces. Echoes are reflected sound waves. Mirrors reflect light waves.
  • Refract: bend when entering a new medium. Light refracts going from air into water.
  • Diffract: spread out around obstacles or through gaps.
  • Interfere: combine when two waves meet. Adding can reinforce; subtracting can cancel.
  • Resonate: build up huge amplitude when a wave matches the natural frequency of an object (this is how a tuning fork sustains a note).
Did you know? All colours of light are simply different wavelengths of the same kind of wave. Red light has the longest wavelength (around 700 nm), violet has the shortest (around 400 nm). Beyond red is infrared (heat radiation). Beyond violet is ultraviolet, then X-rays and gamma rays. Below red are radio waves and microwaves. All of these are electromagnetic waves, just at different wavelengths.

How waves carry energy

Even though the medium does not travel with the wave, the wave does carry energy. A wave on the sea can carry the energy of a distant storm hundreds of kilometres before crashing onto a shore. Sound waves carry the energy of a singers vocal cords across a concert hall. Light waves carry the energy of a star across the universe.

The amount of energy a wave carries depends mainly on its amplitude: bigger waves carry more energy. Loud sounds, bright lights and powerful earthquakes are all high-amplitude waves.

Try this Throw a small stone into a still puddle or pond. Watch how the ripples spread outwards in circles from where the stone hit. Drop two stones close together and watch how the ripples cross over: where two crests meet, they add; where a crest meets a trough, they cancel. This is wave interference, the same principle that lets noise-cancelling headphones silence the rumble of an aeroplane engine by adding an opposite sound wave.
Deeper dive: why everything in physics is a wave (sort of)

In the early 20th century, scientists discovered something extraordinary: everything in the universe, even tiny particles like electrons, can act like waves. This is one of the strangest ideas in physics, called wave-particle duality.

In 1801, the British scientist Thomas Young showed that light passing through two narrow slits forms a pattern of alternating bright and dark bands on a screen behind. This is the classic signature of waves: where two waves arrive in step they add, where they arrive out of step they cancel. Light, it seemed, was definitely a wave.

But in 1905, Albert Einstein showed that light also behaves like a stream of tiny particles called photons. The energy from light kicks electrons out of metal in distinct lumps, not smooth amounts as you would expect from a wave. So is light a wave or a particle? Einstein won a Nobel Prize for showing the answer is "both".

In 1924, the French physicist Louis de Broglie proposed that all particles (electrons, protons, even whole atoms) should have a wave nature too. A few years later, experiments confirmed it: electrons passed through two slits form interference patterns just like light. So do helium atoms, and even whole molecules of buckyball (60 carbon atoms each).

Modern quantum mechanics grew from this discovery. Every particle has a wave-like aspect, with a wavelength inversely proportional to its momentum. The wavelengths of everyday objects (a tennis ball, a person) are unimaginably small, so we never notice the wave nature. But at the scale of atoms and electrons, wave behaviour rules everything. The world is much stranger than it looks at human size.

For more, see what is sound and frequency and pitch.