The Electromagnetic Spectrum

One family of waves

Light, the warmth of a fire, the signal that reaches your phone, the rays at an airport scanner — these all seem unrelated, yet they are the same kind of wave. They are all electromagnetic (EM) waves, and together they form the electromagnetic spectrum.

An EM wave is a vibration of electric and magnetic fields travelling through space. Unlike sound, EM waves need no medium — they pass straight through the vacuum of space, which is why sunlight reaches Earth. You can compare this with mechanical waves like sound in waves and vibrations.

What all EM waves share

Every wave in the spectrum, from the longest radio wave to the shortest gamma ray, has these properties in common:

• They are transverse waves.
• They transfer energy from a source to a receiver.
• They travel through a vacuum at the same speed: the speed of light, about 3 × 10⁸ m/s (300 million metres per second).
• They can be reflected, refracted, and absorbed.

The spectrum, in order

The waves differ in wavelength and frequency. The wave equation links them:

speed = frequency × wavelength

Since the speed is fixed, a longer wavelength must mean a lower frequency, and a shorter wavelength means a higher frequency. Higher frequency also means more energy per wave.

From longest wavelength (lowest energy) to shortest (highest energy):

Wave type	Typical use
Radio waves	TV and radio broadcasting, communication
Microwaves	Cooking, mobile phones, satellite, radar
Infrared	Thermal imaging, remote controls, heating
Visible light	Human sight, photography, optical fibres
Ultraviolet (UV)	Sterilising, security marks, suntan
X-rays	Medical imaging of bones, airport scanners
Gamma rays	Cancer treatment, sterilising equipment

A handy memory aid is: Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma — "Red Monkeys In Vans Use X-ray Guns."

Visible light: the tiny window

Of this enormous spectrum, our eyes detect only a sliver — visible light. Within it, the longest wavelength is red and the shortest is violet, with orange, yellow, green, blue, and indigo in between. White light is a mixture of all of them, which is why a prism splits sunlight into a rainbow. For more on how light behaves, see light and shadows.

More energy, more danger

As you move up the spectrum, the rising energy changes what the waves can do.

• Non-ionising waves (radio through to most visible and lower UV) lack the energy to alter atoms much. Their main risk is heating.
• Ionising waves (higher UV, X-rays, gamma rays) carry enough energy to knock electrons out of atoms. This ionising radiation can damage DNA and cause cell mutations, which is why exposure must be controlled.

That same high energy is also useful: carefully aimed gamma rays can destroy cancer cells, and UV light kills bacteria.

Worked example: frequency of red light

Red light has a wavelength of about 7 × 10⁻⁷ m (700 nanometres). What is its frequency?

Rearrange the wave equation:

frequency = speed ÷ wavelength frequency = (3 × 10⁸ m/s) ÷ (7 × 10⁻⁷ m) frequency ≈ 4.3 × 10¹⁴ Hz

That is about 430 trillion waves every second — and violet light, with a shorter wavelength, has an even higher frequency and carries more energy per wave.

Try it yourself! 🔬

Reveal hidden infrared using a phone camera.

1. Find a TV or air-conditioning remote control and a smartphone with a camera.
2. In a dim room, point the remote at the phone's camera and look at the screen.
3. Press a button on the remote. To your eyes the remote looks dark, but on the screen you may see the LED glow — often pale purple or white.

Many phone cameras pick up near-infrared that your eyes cannot. You have just seen a part of the electromagnetic spectrum that is normally invisible.