Electromagnetic Spectrum

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays look like seven unrelated phenomena. They are one phenomenon at seven different frequencies.

Every electromagnetic wave is a transverse wave consisting of oscillating electric and magnetic fields. None needs a medium, so all of them cross a vacuum — which is how sunlight reaches us. And in a vacuum every one of them travels at exactly the same speed, 3.0 × 10⁸ m s⁻¹.

What differs is frequency, and therefore wavelength, since c = fλ ties the two together at fixed speed. Radio waves can be kilometres long; gamma rays are smaller than an atomic nucleus. Everything else — how they are made, what they pass through, what they do to living tissue — follows from that difference.

c

speed of light in a vacuumm s⁻¹

f

frequencyHz

λ

wavelengthm

The scale is logarithmic because the spectrum spans twenty powers of ten. Drag the marker and watch wavelength and frequency move in opposite directions — as one rises the other falls, because their product is always 3.0 × 10⁸.

Every use follows from a property, and exam answers are marked on the link between the two rather than on the use alone.

Radio waves have the longest wavelengths, diffract around hills and buildings, and are reflected by the ionosphere — so they carry broadcast signals over the horizon. Microwaves pass through the atmosphere with little absorption, which makes them right for satellite communication and mobile phones; they are also absorbed strongly by water molecules, which is what heats food in a microwave oven.

Infrared is emitted by any warm object, so it is used in thermal imaging, remote controls and heaters. Visible light is the narrow band our eyes detect, and is what optical fibres carry.

Ultraviolet is energetic enough to ionise, so it kills bacteria and is used to sterilise water and instruments; it also makes some materials fluoresce, which is how security marks are read. X-rays pass through soft tissue but are absorbed by bone, giving shadow images. Gamma rays, the most penetrating, sterilise equipment and destroy tumours.

The hazard of an electromagnetic wave tracks its frequency, because higher frequency means more energy delivered per photon, and enough energy per photon means the wave can ionise — knock electrons out of atoms in living tissue.

At the low-frequency end the risk is heating. Microwaves are absorbed by water, and since we are largely water, prolonged exposure causes internal heating of tissue. Infrared in excess causes ordinary skin burns.

From ultraviolet upwards the radiation is ionising, and the risk changes character. Ultraviolet damages the surface of the eye and causes skin cancer. X-rays and gamma rays penetrate deep into the body, and can mutate DNA, cause cell damage and cause cancer.

This is why radiographers step behind a lead screen for every exposure. A single X-ray carries a very small risk; taking dozens a day for a career does not.

Almost every long-distance signal you send travels as an electromagnetic wave, and which part of the spectrum is used depends on the distance and the obstacles in the way.

Long and medium radio waves diffract around hills and buildings and are reflected by the ionosphere high in the atmosphere, so they bounce between ground and sky and can be received far beyond the horizon. This is why AM radio reaches remote valleys where television does not.

Shorter wavelengths do not diffract nearly as much, so television and FM radio require a line of sight to the transmitter — hence tall masts on high ground, and a poor picture behind a hill.

Microwaves pass straight through the ionosphere rather than reflecting from it, which sounds like a disadvantage and is in fact the reason they are used for satellite communication: the signal must get out to the satellite and back down again. Mobile phones use microwaves too, communicating with a network of local masts.

Optical fibres carry visible light and infrared instead, and carry far more information than any radio channel. This is why intercontinental internet traffic runs through undersea cables rather than satellites — a fibre has vastly more capacity and much less delay.