Bodies radiate heat through space as infrared radiation and we encounter this radiation most familiarly in this way. Science and technology have many uses for infrared radiation in astronomy, medicine, agriculture, research and in military applications.
Infrared radiation was discovered by the famous astronomer Frederick William Herschel (1738-1822). Herschel was originally a musician. He became fascinated with the mathematics that underlies harmony and this preoccupation led him into astronomy. In 1781 he discovered the planet Uranus.
Herschel damaged an eye making observations of the sun and he took to protecting himself by wearing dark glasses of various colours. He noticed that the heat he felt from the sun varied with the colour of his eye-glass and he decided to measure the temperature of the various colours in light.
Ordinary sunlight (white light) is a mixture of seven different coloured lights. Herschel used a prism to split sunlight into its spectrum of colours - red, orange, yellow, green, blue, indigo, violet. Moving a thermometer from one colour to the next he found that the temperature increased as he moved from the violet through the blue and yellow and on to the red.
Then he found a surprising thing - when he moved the thermometer beyond the red the temperature jumped sharply, whereas when he moved it beyond the violet at the other end of the spectrum there was no rise in temperature. He concluded that invisible heat rays reside beyond the red end of the visible spectrum - the infrared.
We now know that the visible and infrared radiations are members of a family of radiations called the electromagnetic spectrum. The entire family consists of gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, microwave radiation, radio waves, very low frequency radiation, and extremely low frequency radiation. Various members of this family have practical applications, e.g. X-rays (medicine), UV (germicide), microwave (cooking), radio waves (radio), and infrared (see following). Electromagnetic radiation has a dual nature, behaving both as a wave and as a collection of particles (packets of energy called photons).
Let us consider electromagnetic radiation as a wave motion. James Clerk Maxwell (1831-1879), the Scottish mathematical physicist, showed that electromagnetic radiation travels through space at a speed of 186,000 miles per second - the speed of light. A wave is characterised by its wavelength or its frequency. Wavelength is the distance from peak to peak. The electromagnetic spectrum described above is listed in order of increasing wavelength and decreasing frequency.
The wavelength of visible light ranges from 400 nanometres (nm - billionths of a metre) at the violet end to 750 nm at the red end of the spectrum. Infrared light ranges in wavelength from 750 nm to 1,000,000 nm. Frequency means the number of waves per second, or the number of wave peaks that pass a fixed point every second as they rush along at the speed of light. Frequency is measured in cycles per second. One full wave or cycle per second is one Hertz (Hz). Obviously the shorter the wavelength the greater the frequency and vice versa.
When electric charges move, electromagnetic radiation is emitted. Matter is composed of atoms, and atoms contain positive and negative charges. At temperatures above absolute zero, atoms vibrate and radiate electromagnetic radiation at the frequency of their vibration. As temperature rises, atoms vibrate faster.
At the temperatures we encounter in our everyday lives, atoms vibrate at frequencies that emit radiation in the infrared spectrum and we experience this as radiant heat.
Everything we encounter in our everyday world emits infrared radiation - other people, plants, animals, buses, etc. Our eyes can see only a tiny part of the electromagnetic spectrum - the visible part. If we could also see the other parts of the spectrum a whole new world of information would be revealed.
Matter not only emits infrared radiation, but it can also absorb it and is heated in the process. A familiar example is the radiant electric heater. Its heating element glows red hot when a current is passed through it, emitting visible red light and invisible infrared light. The infrared is absorbed by the atoms in our bodies making them vibrate faster and thereby warming us.
Twentieth century science makes much use of infrared radiation. The sun emits a substantial fraction of its energy as infrared radiation but much of this cannot penetrate our atmosphere. Since 1983 astronomers have used satellites above the atmosphere to study infrared emissions coming from the universe at large. This has revealed much about the temperature of stars, distribution of energy in galactic and inter-galactic dust clouds, the atmospheres of planets in our solar system, and the existence of unknown infrared sources.
Infrared photography has been developed to produce very useful information in agriculture and medicine. Aerial or orbital infrared photography allows large-scale monitoring of crop conditions and insect or disease damage to large agricultural areas. It can also be used to detect mineral deposits. Infrared photography is used in medicine to detect various pathogenic conditions that emit more or less heat than normal tissue. Very often those conditions are not detectable by eye and are not recorded on X-ray plates.
And finally, infrared radiation has many military applications. The best known application is the sniper's use of infrared to "see in the dark". In this case, a device on the rifle emits a beam of infrared radiation (dark-light) at the target. The radiation is reflected back and collected by the telescopic sight, which converts it into a visible image.
(William Reville is a Senior Lecturer in Biochemistry and Director in Microscopy at UCC)