Think of a wave – what comes to mind? The ocean swell rocking a boat? Ripples on a pond? The loop moving down a whip as it is cracked? Or perhaps your favourite song pumping out of your stereo speaker? Waves happen when a disturbance (of, say, the flat surface of the water by the ripple, the stillness of the air by a sound wave) moves over time.
Mathematically the simplest and most perfect wave is described by a sine (or cosine) function. You can read a simple introduction to them in Why sine (and cosine) make waves. For example, the function
You can play with this interactive version of a simple sine wave, varying the amplitude, speed and wavelength.
A snapshot in time and space
You might be more familiar with thinking of a sine or cosine function of one variable. You can think of the function
And similarly the function
Many ways of saying the same thing
The quantities of amplitude (), wavelength () and speed () are all we need to completely describe our wave. But there are many equivalent ways of describing our wave with other quantities.
For example, because speed is just distance travelled per unit time, all waves must obey the equation
Then we could rewrite our wave in terms of frequency or period :
However we can rewrite our wave function in much simpler ways with other wave parameters.
You can describe a complete cycle of a wave in terms of the distance it travels in once cycle (the wavelength, ) or the time it takes (the period ). But equivalently you can think of one complete cycle of a wave in terms of angles (you can see why here). One whole cycle of the wave equates to moving through an angle of 360 degrees, or radians. The phase of the wave indicates how far through a cycle the wave is: the phase is 0 radians at the beginning of a cycle, increases over one cycle, passing at the midpoint. At the end of the cycle the phase can simultaneously be thought of as , as it's the end of one cycle, and 0, as it's also the beginning of the next cycle.
Thinking in terms of the phase of the wave can make writing the wave function simpler. The angular frequency of the wave is
Similarly the angular wavenumber (often just called the wavenumber) of the wave is
Now you can write our wave function in this much simpler form:
The wave function we gave above is a solution to the wave equation:
Waves arising in nature can be complex
Wave equations like this one arise in lots of physical situations, from the sound made by musical instruments to the seismology of stars. Other more complex wave equations arise in the study of fluid mechanics, such as the Euler equations for the movement of water under an ocean wave and the Navier Stokes equations, and even in quantum physics. In these more complex wave equations the wave function that explicitly defines the position of a point on the wave at any time is harder, or even impossible to find. In these cases methods for approximating the solution are used to find a function that closely matches the desired behaviour of the wave, using techniques from computational fluid dynamics.
About this article
This article was produced as part of our coverage of the Dispersive hydrodynamics: mathematics, simulation and experiments, with applications in nonlinear waves programme hosted by the Isaac Newton Institute for Mathematical Sciences. You can find more content about the programme here.
Rachel Thomas is Editor of Plus.
This article was produced as part of our collaboration with the Isaac Newton Institute for Mathematical Sciences (INI) – you can find all the content from our collaboration here. The INI is an international research centre and our neighbour here on the University of Cambridge's maths campus. It attracts leading mathematical scientists from all over the world, and is open to all. Visit www.newton.ac.uk to find out more.