What is a wave?

Far from shore a gull rides up and down as each wave passes, yet it does not travel with the wave.

A disturbance that travels

Drop a stone into a still pond and rings spread out across the surface. Pluck a guitar string and it blurs into a shimmer. Shout across a field and someone hears you a moment later. All three are waves. OpenStax College Physics gives a short definition: a wave is a disturbance that propagates, or moves away from the place where it was created. In the pond the disturbance is the shape of the water surface. In the air it is a change in pressure. On the string it is a sideways bend.

The important word is disturbance. The pattern moves, but the material mostly stays where it was. A gull sitting on the open sea goes up as a crest arrives and down as a trough passes, and after the wave has gone by it is roughly where it started. OpenStax notes that in water waves far from shore, the water particles move in small circles and stay in one area while the energy moves on. If a wave pushes you in deep water, what you feel is the energy, not a stream of water rushing past.

Crest, trough, wavelength and amplitude

A simple repeating wave has high points called crests and low points called troughs. The distance from one crest to the next, measured along the direction the wave travels, is the wavelength, written with the Greek letter λ (lambda). The amplitude is how far the medium moves away from its resting position, so it is the height of a crest above the flat line, or half the distance from crest to trough. OpenStax points out that the amplitude is completely independent of how fast the wave travels. A big wave and a small wave on the same rope move at the same speed.

Two directions of shaking

Waves come in two basic kinds, sorted by the direction of the shaking compared with the direction of travel. In a transverse wave the medium moves at right angles to the direction the wave goes. Flick one end of a rope tied to a post and a hump runs along it while each bit of rope moves up and down. Waves on the strings of musical instruments are transverse, and so are light and other electromagnetic waves. In a longitudinal wave the medium moves back and forth along the direction of travel. Push and pull one end of a long spring and you see bunched-up regions, called compressions, run along it.

Sound in air and in water is longitudinal. A fluid has almost no stiffness against being bent sideways, so the only disturbance it can pass along is squeezing and stretching. Solids can carry both kinds. Water waves on the sea are a mix of the two, which is why the water particles trace circles. Earthquakes send both kinds through the ground: compressional P-waves that travel at 4 to 7 km/s and shear S-waves at 2 to 5 km/s, according to OpenStax. Seismographs record the P-wave first, and the gap before the S-wave arrives tells how far away the earthquake was.

Waves that need no medium

Water waves, sound and earthquake waves are mechanical waves: they need some material to travel through. Sound cannot cross empty space because there is nothing to squeeze. Light, radio and the other electromagnetic waves are different. They are made of changing electric and magnetic fields and travel through a vacuum, which is how sunlight reaches us across 150 million kilometres of space. OpenStax warns about a common mix-up here: a radio carries sound to you, but the radio waves themselves are electromagnetic, not sound.