The Electromagnetic Spectrum

One family of waves: what all electromagnetic waves have in common

Electromagnetic radiation is a way of transferring energy from a source to an absorber without any matter in between. Every region of the electromagnetic spectrum - radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays - is made of oscillating electric and magnetic fields, and every one of them is a transverse wave.

The most important shared property is speed. All electromagnetic waves travel at the same speed in a vacuum, 3.0 x 108 m/s, and at approximately the same speed in air. Sound cannot cross empty space because it needs a medium to vibrate, but electromagnetic waves need no medium at all, which is why light and radio signals reach us from space.

The regions differ only in frequency and wavelength, and the two are locked together by the wave equation: speed = frequency x wavelength, or v = f x wavelength. Because the speed is fixed, a wave with a high frequency must have a short wavelength, and a wave with a low frequency must have a long wavelength.

Worked example. A microwave oven uses waves of frequency 2.45 x 109 Hz. Rearranging the equation gives wavelength = v / f = 3.0 x 108 / 2.45 x 109 = 0.12 m. So the waves inside the oven are about 12 cm apart, which is the same order as the size of the food.

The order of the spectrum, from long radio waves to short gamma rays

Learn the order in one direction and you can write it in the other by reversing it. In order of increasing frequency the regions are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. In order of increasing wavelength the list is exactly the reverse: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves.

The regions are not separated by hard walls - the ranges overlap and blend into one another - but each has a range of wavelengths that examiners expect you to recognise. Radio waves are the longest, from about a metre up to many kilometres. Gamma rays are the shortest, with wavelengths smaller than the nucleus of an atom.

Visible light is the narrow strip in the middle that our eyes can detect. Within it, the colours run from red at the long wavelength end (about 700 nm) through orange, yellow, green, blue and indigo to violet at the short wavelength end (about 400 nm). Remember the order with red, orange, yellow, green, blue, indigo, violet.

A good way to picture the whole spectrum is the energy carried by the waves. Frequency rises and wavelength falls as you move from radio to gamma, and the energy each wave carries rises with it. That is why radio waves are harmless at everyday intensities while gamma rays must be handled with great care.

Radio waves and microwaves: the long wavelength end

Radio waves have the longest wavelengths and the lowest frequencies, and they carry the least energy of any region. They are used for radio and television transmissions: the programme is carried on a radio wave sent out from a transmitter and picked up by an aerial. Radio astronomy uses large dishes to detect the faint radio waves given out by stars, galaxies and clouds of gas; unlike visible light, radio waves pass through cloud and can be collected day or night. RFID, radio frequency identification, uses a small tag that reflects radio waves back to a reader, which is how shop security tags, contactless cards and library books are identified.

Microwaves are shorter than radio waves, roughly from 1 mm to 1 m, and they are used for satellite television, mobile phones and microwave ovens. In a microwave oven the waves are absorbed by water molecules in the food, which makes them vibrate faster, so the food heats up from the inside. Mobile phones and wireless internet use microwaves because microwaves can penetrate some walls and only require a short aerial for transmission and reception, which is what makes a handheld phone possible.

Communication with artificial satellites is mainly by microwaves. A low orbit satellite is close to the Earth and moves quickly across the sky, so an aerial has to track it and the link is only available for a few minutes at a time; some satellite phones use a constellation of these. A geostationary satellite orbits once every 24 hours above the equator, so it appears fixed above one point on the Earth and a dish can point at it permanently. Some satellite phones and direct broadcast satellite television use geostationary satellites for this reason.

Radio waves behave differently in buildings from microwaves. Bluetooth uses radio waves because radio waves pass through walls, but the signal is weakened on doing so, so the range between devices is short.

• Radio waves: radio and television transmissions, astronomy, RFID tags.
• Microwaves: satellite television, mobile phones and wireless internet, microwave ovens.
• Microwaves penetrate some walls and need only a short aerial, so phones can be small.
• Low orbit satellites move across the sky; geostationary satellites stay fixed above one point.

Infrared and visible light

Infrared lies just beyond the red end of the visible spectrum. Any warm object gives out infrared, and the hotter it is the more it gives out, so infrared is the region of heat radiation. Electric grills and toasters use infrared to heat food directly without heating the air around it. Remote controllers for televisions send a narrow beam of infrared a few metres to the receiver, an example of short range communication. Intruder alarms use passive infrared sensors to detect the body heat of a person moving through a room. Thermal imaging cameras turn the infrared given out by an object into a picture, which lets firefighters see through smoke and surveyors spot heat escaping from a building. Optical fibres carry infrared or visible light along a thin glass strand.

Visible light is the only region our eyes can detect, and its uses follow directly from that: vision, photography and illumination. The colour of the light depends on its wavelength, from red at about 700 nm to violet at about 400 nm.

Optical fibres are used for cable television and high-speed broadband. Glass is transparent to visible light and to some infrared, so the signal is not absorbed as it travels along the fibre, and the light is kept inside by total internal reflection so it follows the bends in the cable. Visible light and short wavelength infrared have high frequencies, so they can carry high rates of data - far more than a copper cable or a radio link of the same size.

• Infrared: electric grills, remote controllers, intruder alarms, thermal imaging, optical fibres.
• Visible light: vision, photography, illumination.
• Optical fibres carry visible light or infrared by total internal reflection.
• Glass is transparent to visible light and some infrared, and short wavelengths carry high rates of data.

Ultraviolet, X-rays and gamma rays: the short wavelength end

Ultraviolet sits just beyond the violet end of the visible spectrum. It is used for security marking, where a mark that is invisible in ordinary light glows when ultraviolet is shone on it, and for detecting fake bank notes, because genuine notes carry marks that fluoresce under an ultraviolet lamp while many forgeries do not. It is also used for sterilising water: ultraviolet damages the genetic material of microorganisms so that they cannot reproduce, which kills them without adding chemicals to the water.

X-rays have very short wavelengths and very high frequencies. They are used for medical scanning: X-rays pass through the soft tissue of the body but are absorbed much more strongly by the denser calcium in bone, so the bone casts a shadow on the detector and shows up as a bright image. Security scanners at airports work in the same way, using X-rays to see the shapes of objects inside luggage and sealed cases.

Gamma rays have the shortest wavelengths, the highest frequencies and the greatest penetrating power of all. This makes them ideal for sterilising food and medical equipment: a dose of gamma rays passes through the packaging and kills bacteria and other microorganisms without heating the product, so instruments can be sterilised inside a sealed packet. In medicine, gamma rays are used for the detection of cancer, with a gamma camera picking up a small amount of a gamma-emitting tracer that has collected in a tumour, and for its treatment, where beams of gamma rays are aimed at the tumour to kill the cancer cells.

• Ultraviolet: security marking, detecting fake bank notes, sterilising water.
• X-rays: medical scanning, security scanners.
• Gamma rays: sterilising food and medical equipment, detection and treatment of cancer.
• Gamma rays are the most penetrating, so they can sterilise equipment inside sealed packaging.

The harmful effects of excessive exposure

Electromagnetic waves transfer energy to whatever absorbs them, so a large enough dose can damage the body. The harm depends on the region, because different regions are absorbed by different parts of the body. Microwaves are absorbed by water inside the tissue and cause internal heating of body cells; the body cannot easily cool deep tissue, so this is more dangerous than a burn on the surface. Infrared is absorbed by the skin and causes skin burns, in the same way as sitting too close to a fire.

Ultraviolet is absorbed by the surface layers of the skin and by the eye, and it damages the cells there. This leads to skin cancer and to eye conditions such as cataracts, as well as sunburn and the painful inflammation of the eye called snow blindness. Ultraviolet is the reason for the ozone layer and for sunscreen.

X-rays and gamma rays are ionising radiation. Their waves carry enough energy to knock electrons out of atoms, which can break the chemical bonds in DNA and other molecules. This causes mutation or damage to cells in the body, which can lead to cancer many years later, so radiographers stand behind a screen, wear lead aprons and keep the exposure time as short as possible.

The pattern to remember is that the higher the frequency, the more energy each wave carries and the more damaging it can be. Ultraviolet, X-rays and gamma rays are ionising and can damage cells directly; microwaves and infrared are not ionising, but they can still cause serious heating and burns at high intensity.

Digital and analogue signals

Information such as a sound can be carried as either an analogue or a digital signal. An analogue signal varies continuously and can take any value within a range, so its graph is a smooth curve that follows the original sound wave. A digital signal has only two values, usually written as 0 and 1, or low and high, so its graph is a series of pulses. A microphone produces an analogue signal, which can be converted into a digital one before it is transmitted, and a sound can be sent by either method.

Digital signalling has two big advantages. The first is an increased rate of transmission of data: because the signal is a stream of pulses, many separate pieces of information - several telephone calls, television channels or streams of computer data - can be sent along the same path at once, and a high frequency carrier such as visible light in an optical fibre can carry an enormous number of pulses each second.

The second advantage is an increased range due to accurate signal regeneration. Every signal picks up noise as it travels. If an analogue signal is amplified, the noise is amplified with it and the shape of the wave is spoilt a little more at each repeater, so the signal gradually becomes unreadable. A digital signal only has to be recognised as a 0 or a 1, and even a distorted pulse can still be identified as one of those two values. The receiver can therefore rebuild a clean pulse, so the noise does not build up and the signal can travel much further before it fails.

This is why television, radio and telephone systems have largely moved from analogue to digital. Digital signals give more channels, better quality and a longer range from the same transmitters.

• Analogue signal: varies continuously and takes any value in a range.
• Digital signal: only two values, 0 and 1, so it is a series of pulses.
• A sound can be transmitted as an analogue signal or as a digital signal.
• Digital gives a higher rate of data transmission and a greater range, because the signal can be regenerated accurately.