19.9 Electromagnetic waves

In post 16.25, we saw that an electric charge produces an electric field. If the charge oscillates, relative to an observer, then the field will oscillate. We also saw that a moving electrical charge produces a magnetic field. If this movement is oscillatory, the magnetic field will be oscillatory. The fields can have an influence far from the oscillating charge. This combination of an oscillating electric field and an oscillating magnetic field is called electromagnetic radiation. Its properties depend on the frequency, f (post 18.10), of oscillation.

It takes a finite time for electromagnetic radiation to travel. In a vacuum, this speed, c, does not depend on f, and has a value of 3.00 × 108 m.s-1 (see post 16.7 if you don’t understand this way of writing numbers and the units). So, we can think of electromagnetic radiation as waves whose speed is given by their frequency multiplied by their wavelength, λ (post 18.10). Then, for an electromagnetic wave

c = fλ.

The picture below represents an electromagnetic wave, at an instant in time.

fig1

It is important not to think of the electric and magnetic fields as being independent entities – they are both a consequence of a charge oscillating at a distance. You can’t have one without the other. Notice that the vectors representing the electric and magnetic fields (post 17.24) are perpendicular to each other and to the direction of propagation of the wave – an electromagnetic wave is an example of a transverse wave (post 18.10). In the picture above, each vector oscillates in a fixed plane. If these planes remain the same, the wave is said to be polarised.

It is also important to realise that, unlike a water wave (post 18.10) or a wave on a stretched string (post 18.12) an electromagnetic space does not require oscillation of a physical object to propagate. This means that it can propagate through empty space. Before the twentieth century, people found this idea puzzling, so they had to invent a mysterious substance called the ether (or aether) that enabled the waves to propagate.

We classify electromagnetic radiation according to its properties. This classification started before people knew anything about electromagnetic radiation. We can see one form (light) and feel another (infra-red radiation) as the heat from a glowing object.

The table below gives the frequencies of the different forms of electromagnetic radiation. If you are not sure what 1019, and so on, mean – see post 18.2. The unit of frequency is the hertz (Hz) defined in post 16.14.

fig2

The region between 0.43 and 0.75 × 1014 Hz, is shown in more detail below with the colours that we see at different frequencies. It also shows the corresponding wavelengths calculated from the value of c, using the equation in the second paragraph (above); here distances are measured in nanometers (nm, post 16.12).

fig3

In the same way that pitch is the frequency of mechanical oscillation detected by our ears (post 18.13), so colour is the frequency of electromagnetic oscillation detected by our eyes.

If you’ve read this post to the end, you may think that I have told you some things without really explaining them. This is because its sometimes easier to know a bit about something before we explore it further (see post 17.25). So, I hope to return to some of this stuff in later posts.

 

Related posts

19.8 Wave energy
18.24 Analogies between electrical and mechanical systems
18.23 Frequency response and resonance
18.13 Sound
18.12 Vibrating strings
18.11 Motion in a circle, the simple harmonic oscillator and waves
18.10 Waves
17.24 Fields and vectors
16.25 Electrical charge

Follow-up posts

20.28 Polarised light

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