The Electromagnetic Spectrum

Programme Outline

Jason Bradbury uses the theme of 'searching' to show us some modern applications of parts of the electromagnetic spectrum. The main regions of the electromagnetic spectrum that are used for searching are:

• X-rays: used by customs at the border between Hong Kong and China to penetrate lorries and look for contraband.
• Infrared: used by the police to detect people who are trying to hide from them.
• Microwaves: used by SETI to find messages that might have been sent by intelligent extraterrstrial life.

The programme uses the idea that extraterrestrial life might be watching us through all the television and radio signals that are beamed out (or leak out) from the Earth and are radiated into space.

00.00 - 01.05 Making waves

We meet Jason as a 'Man in Black'. He is wondering about extraterrestrials and how they would contact us. They cannot use sound because it doesn't travel through the vacuum of space. Instead they would use part of the electromagnetic spectrum - perhaps radio waves or microwaves.

01.05 - 02.24 Electromagnetic spectrum

Like the notes on a keyboard, the electromagnetic spectrum is made up of a range of frequencies. A graphic of the whole electromagnetic spectrum is used to illustrate that high frequencies have short wavelengths and low frequencies have long wavelengths.

02.24 - 3.42 Radio spectrum

Which part of the radio spectrum would extraterrestrials use? A lot of our radio programmes are broadcast in the VHF part of the spectrum. However, we can't receive these waves from space because they don't get through the atmosphere. Only waves with certain frequencies can get through the 'window in the atmosphere'. This window is from about 300 to 3,000,000 MHz (wavelengths from 1 m to 0.1 mm). These are mainly in the microwave ragion of the spectrum.

03.42 - 05.10 Search for Extra-Terrestrial Intelligence (SETI)

SETI is an organisation which monitors the signals coming from space. They use volunteers all round the world so that they can pick up microwave radiation from all directions. There is a station in the UK run by Trevor Unsworth, who has a parabolic dish in his back garden in Sheffield. Using this dish and amplifiers, he can detect signals from up to 400 light years away.

05.10 - 05.30 The light year

A light year is a measure of distance. One light year is the distance light travels in one year. Light travels at about 3 x 10⁸ m/s. So the distance it travels in one year is 3 x 10⁸ x 60 x 60 x 24 x 365 = 9 x 10¹⁵ m - approximately one million million kilometres.

05.30 - 07.25 More from SETI

We look at the SETI equipment. 'Alien spotters' monitor the whole microwave region of the spectrum right around the clock. A radio scanner automatically tunes through all the frequencies. The signal level is digitalised and recorded on a chart. This shows a constant background radiation. A larger signal may represent a message. One such signal was detected in 1977.

07.25 - 11.09 Television transmissions

As well as receiving messages from outer space, we are constantly sending messages out into space. This isn't always deliberate: many are signals (usually from television) that escape from the Earth and radiate out. An intelligent life form 200 light years away could be watching Science in Focus in 200 years' time. You can imagine waves of history spreading out from the Earth.

Although we broadcast television signals in the UHF region of the electromagnetic spectrum, they are carried around the country by microwaves. A programme will be turned into a digital signal, which is carried by optical fibres (using infra-red radiation) to the BT Tower in London. Here, the signal is put onto a microwave carrier, which is beamed across the country. Microwaves are 'line of sight' waves: they will not bend around the curve of the Earth or travel through large buildings. Therefore, they hop between repeater stations that are about 100 km apart. These carry the microwaves in stages to their destination transmitters, which broadcast the signals on a UHF wave.

11.09 - 12.13 Satellites around the world

Television signals are also sent around the world via satellites. These are like extremely tall transmitter masts giving a one-stop line of sight to another part of the world. Once again, microwaves are used to carry the signals, partly because they can get through the Earth's atmosphere. The microwaves are sent up from Goonhilly in Cornwall. The satellite is a very small target as it is 1500 km away. It receives only 1% of the wave that is sent up; the other 99% goes straight past the satellite and into space. These lost waves could conceivably make contact with an extraterrestrial intelligence.

12.13 - 14.20 X-rays

We know that X-rays can be used to 'see' inside people's bodies. In this sequence, we discover how the customs officials on the border between Hong Kong and China are using X-rays to look inside lorries. These X-rays are 50 times more powerful than those used in hospitals, so that they can penetrate the lorry's steel casing and reveal structures within. As with hospital X-rays, they reveal differences in density because they are absorbed differently by different materials.

14.20 - end Infrared

The final sequence shows an infrared camera being used by the Metropolitan Police to track down criminals. Anything warm gives off infrared radiation. This means that visible light is not necessary to detect a person. There is even a facility to add false colour to the image to highlight the different temperatures. This can be a rainbow of colours; or an 'iron-bow', which is is based on the colours in which iron glows when it is heated (so red is hot and white is very hot). A colour chart can be used to determine the temperatures.

The infrared camera is used to show the temperature changes of a cup of tea and of someone drying his hair.