The Basics of Waves

Before you learning about waves make sure you can define the quantities amplitude, wavelength and frequency. Without secure definition of those things then you're going to find all of this topic pretty difficult to follow. Go to the definitions page and ensure that you can know them before you read on.

Amplitude and wavelength are best shown on a diagram, like this one. The amplitude can be described as the height of the wave from the equilibrium position to the peak. And the wavelength usually from peak to peak, but really it would be correct to say trough to trough, or indeed any point on one wave cycle to the corresponding point on the next wave cycle.

Frequency is not easily shown on a diagram, so that's best defined in another way. Try and understand frequency as "how frequent the waves are". Imagine water waves crashing onto a beach, count how many land in a minute, let's say ten. That would be a frequency of ten per minute.

Let's take our example a little further, if you were to measure the time period of those same waves, it would take six seconds to do one full wave, from peak to trough back to peak again. That makes sense right? Ten in a minute, so six seconds each. That leads us to a useful definition of frequency, one that it pays to remember. Frequency is inverse the time period, or in an equation f=1/T. So that same frequency can be expressed as a one sixth of a wave per second. Per second is in fact our usual unit for frequency, as we'll usually be dealing with much higher frequencies of wave. But we give the unit the name Hertz, or just Hz, after Heinrich Hertz.

Students often muddle frequency and speed, they'll say frequency is how fast a wave is. This is incorrect as travelling waves have a speed like any other moving thing, be careful to make this distinction, because an examiner can only give you a mark for describing frequency in terms of numbers of wave cycles per second. Check out the next page on how we work with wavespeeds.

Probably you've come across the idea that the pitch of sound is related to its frequency, or that human hearing ranges between 20Hz and 20kHz.

Maybe you already know that the frequency of light is what determines the colours that we perceive, or that there is a whole lot more light in the electromagnetic spectrum, light that we cannot see, and that that spectrum ranges from around 100Hz all the way up to 1019Hz.

In any case as long as you've understood all this, then you've got the basics that you'll need to understand one of the most fascinating topics in Physics.

© Kit Betts-Masters kit@gorillaphsyics.com