The electromagnetic spectrum

What is light?

This question might well be posed by an interested pupil. The approach taken in this topic starts on familiar ground with light being given out by the Sun, travelling in straight lines, and allowing us to see. The idea that light is some kind of wave motion is quite common, but these wave ideas add little (if anything) to the physics narrative at this level. Nevertheless if you are interested in going deeper, read on for more about the electromagnetic spectrum. The SPT: Radiations and radiating topic is much more comprehensive.

When we refer to the visible light that is given out by the Sun and that allows us to see the world around us, we are referring to one part of the electromagnetic spectrum. The spectrum of visible colours that you will be familiar with is just one small part of a huge family of radiations, ranging from radio waves to gamma rays.

Teacher Tip: You'll also meet spectra in the SPT: Sound topic

Properties of electromagnetic radiation

These various kinds of radiation have quite different properties, so what is it that they all have in common which allows us to say that they are part of the same family?

The physical nature of the radiation is the same throughout the spectrum. We refer to electromagnetic radiation because all parts of the family consist of linked electric and magnetic fields that oscillate perpendicular to each other. The radiation moves in a direction perpendicular to both of the fields.

As a result all of these radiations have common behaviours, so they're referred to as a family. One common behaviour is that they all travel at the same speed in a vacuum. All of these electromagnetic waves (whether radio, visible or gamma) travel at the same speed. They all travel at the 'speed of light' through a vacuum, that is at 3 × 10⁸ m s^-1.

The significant difference from one part of the spectrum to another is in frequency.

Electromagnetic radiation consists of linked, oscillating electric and magnetic fields.

Radiation as waves

Electromagnetic radiation, such as visible light, is wave-like in nature because it consists of these oscillating electric and magnetic fields.

While water waves are made up of particles, moving up and down at right angles to the direction of travel of the wave, electromagnetic waves consist of oscillating fields. If you think that this is a difficult idea to get hold of, you'd be right! It is one matter to think of water moving up and down to create a wave, but something quite different to imagine oscillating fields (since a field is itself an abstract mathematical idea). (If you ask any watersports enthusiast, they'll tell you that the particles in water waves don't only move up and down, but any additional movements are small compared to the up and down movement. The approximation is helpful here.)

Since radiations from all parts of the spectrum travel at the same speed, and the frequencies vary, the wavelengths of the different parts of the electromagnetic spectrum must also be different.

The differences in wavelength can be huge. For example, radio waves might have a wavelength of 1500 metre (the 'metric mile'), while X-rays might have a wavelength of 1 × 10^-10 m (close to the separation between molecules in solids).

Frequency, speed and wavelength

Here are the quantities, and how they're measured: frequency is measured by counting the number of cycles per second in hertz; wavelength is the shortest distance between points of the wave in step in metres; speed is the distance the wave travels each second in metres / second.

For any wave (whether water, light, sound), there is a relationship between the wave speed, frequency and wavelength.

Here's a precise way of writing it out, so that every term is just a number:

Wavespeedmetre/second = frequencyhertz × wavelengthmetre

You can also write (making notes to yourself about the units):

wavespeed measured in metre/second = frequency measured in hertz × wavelength measured in metre

Or even express it rather concisely as:

wavespeed = frequency × wavelength

Since all of the radiations of the electromagnetic spectrum share the same speed, it follows that radiations with a relatively high frequency (such as light) must have a relatively short wavelength and vice versa. Here are some selected values: radio waves have a frequency of 2 × 10⁵ Hz and a wavelength of 1.5 × 10³ m; visible light has a frequency of 3 × 10¹⁵ Hz and a wavelength of 1 × 10^-7 m; X-rays have a frequency of 3 × 10¹⁸ Hz and a wavelength of 1 × 10^-10 m .

For now, we suggest that you concentrate on the fundamentals of frequency and amplitude. Wavelength can come later.