TWO HUNDRED YEARS OF DETECTING THE INFRARED
February 11th marks the 200th anniversary of the day when the famous astronomer Sir William Herschel discovered infrared rays, while he was studying sunlight. Today, infrared radiation from celestial objects provides some of the most important information about the universe available to astronomers. And infrared radiation and infrared detectors are used in a vast range of applications for the benefit of society.
In 1800 Sir William Herschel (who also discovered the planet Uranus, in 1781) published the results of a series of experiments he had carried out 'on the heating powers of coloured rays'. Herschel spread sunlight out into a spectrum (or rainbow) and placed thermometers into the light of different colours. He noticed that red light gave him the greatest temperature rise, but was most surprised to find that a thermometer placed off the red end of the spectrum where no light was visible recorded an even higher temperature. He deduced that the thermometer was registering invisible radiant heat, what he called 'calorific rays'. Herschel went on to prove that this radiant heat - what we now call infrared radiation - obeyed the same laws of reflection and refraction as visible light.
Infrared astronomy has come a long way in the 200 years since those early observations of the Sun by Herschel. Most infrared radiation from space cannot be measured from Earth's surface because the atmosphere absorbs so much. Astronomers overcome this problem by building telescopes on high mountains in very dry places, by flying balloon experiments, and most recently by using satellites, such as the Infrared Space Observatory (ISO) launched in 1995. The UK Infrared Telescope (UKIRT) is located on Hawaii, at an altitude of almost 14,000 feet (4,200 metres). Though infrared is absorbed by our atmosphere, it can travel through the interstellar dust that prevents us from seeing many parts of our own Milky Way and other galaxies. Using infrared detectors astronomers can reveal hidden parts of the universe, such as the dark clouds where new stars are being created.
Infrared images of the universe can be every bit as impressive as the more familiar visible ones. Dr Helen Walker of the Rutherford Appleton Laboratory has assembled an exhibition to celebrate the anniversary. "The new pictures of Orion from UKIRT were so beautiful, I just sat there admiring them for a while, forgetting I was in a rush to download them!" she said, "and when I first saw the video of the Milky Way as seen by the MSX satellite my jaw dropped - it was so stunning".
But detecting infrared rays does not only benefit astronomers. For example, detectors able to pick up infrared given off by the warmth of a human body, have many practical applications. Fire and Rescue services use them to find people buried under collapsed buildings, and similar cameras are used by other emergency services world-wide. Many burglar alarms use infrared detectors to sense the heat from a person entering a room and act as their trigger. Earth observation satellites, such as ESA's ATSR satellites, make use of the infrared to monitor sea surface temperatures, and can track the build up of events such as El Nino.
After a private view on 11th February for members of the Royal Astronomical Society, the exhibition organized by Helen Walker will tour the country, including visits to Herschel House Bath, Slough Museum, Imperial College London, and Rutherford Appleton Laboratory near Oxford.
Contact: Dr Helen Walker
Useful web addresses for pictures
http://www.nmsi.ac.uk/on-line/treasure/objects/1876-565.html - Herschel's prism and diagram
http://www.ipac.caltech.edu/ipac/iras/iras.html IRAS pictures
http://www.iso.vilspa.esa.es/science ISO pictures
http://gibbs1.plh.af.mil/ - MSX pictures
http://www.jach.hawaii.edu/JACpublic/UKIRT/public/gallery.html UKIRT pictures
http://www.atsr.rl.ac.uk ATSR sea surface temperature pictures