What is a wave?

Waves carry energy

A wave is a traveling disturbance that carries energy from one place to another. When you drop a pebble into a pond, ripples spread outward. Water moves up and down locally, but the water itself does not travel all the way across the pond with the ripple. The energy of the disturbance travels outward.

Waves appear in many forms: water waves, sound waves, light waves, seismic waves, waves on strings, radio waves, and waves in springs. Although these examples look different, they share common features.

Medium or no medium

Some waves need a material medium. A medium is the substance through which a wave travels. Sound needs air, water, or some other material because it is a vibration of particles. Water waves need water. Waves on a rope need the rope.

Light is different. Light is an electromagnetic wave, and it can travel through empty space. This is why sunlight reaches Earth across the vacuum of space. Sound from the Sun does not reach us because sound cannot travel through vacuum.

Transverse and longitudinal waves

In a transverse wave, the disturbance is perpendicular to the direction the wave travels. A wave on a rope is a good example. If the wave travels horizontally along the rope, the rope moves up and down.

In a longitudinal wave, the disturbance is parallel to the direction the wave travels. Sound in air is longitudinal. Air molecules compress and spread out along the direction the sound travels. Regions where particles are closer together are compressions, and regions where they are farther apart are rarefactions.

Wavelength, frequency, and amplitude

Wavelength is the distance between repeating parts of a wave, such as crest to crest or compression to compression. It is often represented by $lambda$, the Greek letter lambda.

Frequency is how many wave cycles pass a point each second. It is measured in hertz, written $Hz$. One hertz means one cycle per second. A higher frequency means more cycles each second.

Amplitude measures the size of the disturbance. For a water wave, amplitude relates to wave height. For sound, amplitude relates to loudness. For light, amplitude relates to intensity or brightness.

Wave speed

Wave speed connects wavelength and frequency:

$v = flambda$

Here $v$ is wave speed, $f$ is frequency, and $lambda$ is wavelength. If frequency increases while wave speed stays the same, wavelength decreases. This relationship appears in sound, light, water waves, and many other wave systems.

The speed of a wave depends on the type of wave and the medium. Sound travels faster in water than in air. Light travels fastest in vacuum and slower in materials such as glass or water.

Reflection, absorption, and transmission

When a wave reaches a boundary, several things can happen. It may reflect, bouncing back. It may be absorbed, transferring energy into the material. It may be transmitted, passing through into another material. Often all three happen partly.

Echoes are reflected sound waves. A dark shirt absorbs more visible light than a white shirt. A window transmits much visible light while reflecting some.

The big idea

Waves are a major way energy moves through the universe. They can travel through matter, and some can travel through empty space. By understanding wavelength, frequency, amplitude, and wave speed, we gain a common language for sound, light, water waves, earthquakes, music, communication, and modern technology.