Electromagnetic Waves (overview)

We think of light as being special – but that’s only because our eyes respond to it, allowing us to see the world around us. In fact, light is just one example of an electromagnetic wave. Other electromagnetic (EM for short) waves include infra-red, ultra-violet, radio and x-rays.

The range of EM waves, known as the electromagnetic spectrum, is shown in the diagram below. Note that the spectrum does not have fixed boundaries at its ends: radio waves extend beyond 10 000 km and (cosmic) gamma rays can be shorter than 10 ^-14 m.

This topic (electromagnetic waves) is one that makes plentiful use of unit prefixes and scientific notation, both of which appear in the paragraph above. Another prefix, nanometres (nm, or 10 ^-9 m) appears in the paragraph below, so make sure that you’re confident with conversions as they could easily come up in examination questions!

Despite the importance that we attach to it, visible light forms the smallest part of the electromagnetic spectrum, covering wavelengths from approximately 650 nm to 350 nm. You are expected to be able to name all the common EM wave types and put them in the correct order (three to each side of visible light). But what, you may ask, decides the order? The answer is wavelength.

You should recall that all waves are defined by the distance between their adjacent peaks: this distance is the wavelength, measured in metres. You should also know that waves have another important property, which is that rate at which they go “up and down”: this is called the frequency and is measured in hertz (Hz). Each complete “up and down” of the wave is called a cycle or oscillation. (In times gone by, wave frequency used to be measured in cycles-per-second, which is more obvious than hertz, but you must use hertz as this is the correct SI unit.)

If we generate 100 cycles every second then the frequency is 100 Hz. And if the distance between each successive wave peak is 25 cm (0.25 m) then the wave must extend by 100 x 0.25 metres every second. In other words, the front of our wave will move forwards by 25 metres each second. You should spot that the calculated forward movement is actually a speed (25 m/s). This connection between wave characteristics can be written as a mathematical relationship, called the wave equation, that you must be able to use;

Importantly, all EM waves travel at exactly the same speed, which is often stated as “the speed of light”. The wave equation also applies to non-electromagnetic waves, including sound and water waves, but the wave speed in those cases is much slower.

Now let’s return to putting EM waves in the correct order by arranging them with decreasing wavelength, as shown in the diagram at the top of this article. Note that the wave equation tells us that reducing the wavelength is equivalent to using increasing frequency.

There is a useful mnemonic that you can use to remember this order;

Despite their different wavelengths and frequencies, all electromagnetic waves have the following important properties in common;

• they propagate (travel) at the same speed, which is 300 000 000 m/s (3 x 10⁸ m/s)
• they are transverse waves, which means the cycles go “up and down” when the wave front moves forwards (the oscillations are perpendicular to the direction of travel)
• they propagate through “free space” without the need for any medium, unlike sound (which requires air/gases/solids) or water waves (which require water, of course)

Although all EM waves have these things in common, the various wavelengths of EM waves give them different behaviours, and these differences lead to the labels that are applied to the different types of EM waves.

Note that there is no firm boundary between the different types of waves. Microwaves, for example, can be used for radio communication (in smartphones) as well as for heating (in microwave ovens) so microwaves clearly blur into both of the adjacent bands.

There will be more detail about the behaviours (including uses and dangers) of different types of EM waves in the next article. In the meantime, you can download a summary of this article and some brief notes about how IR and UV waves were discovered, here. For a more interactive summary, watch the video embedded at https://science.nasa.gov/ems/01_intro.