These rays were discovered in 1800 by William Herschel, a British musician and astronomer, when he observed that a thermometer placed just outside the visible spectrum of sunlight shows a greater increase in temperature than one placed in the red region.
The Infrared region of the spectrum lies beyond the red end of the visible range, with wavelengths between 0.01 to 7.5x10-5 cm.
Instruments for detecting infrared radiation include heat-sensitive devices such as thermocouple detectors, bolometers, photovoltaic cells, and photoconductors.
Infrared radiation is absorbed and emitted by the movement (rotations and vibrations) of chemically bonded atoms or groups of atoms of many materials. Some of the materials that absorb infrared radiation are window glass, water and also our atmosphere. Although invisible to the eye, longer infrared radiation can be detected as warmth by the skin. It forms nearly 50% of the Sun's radiant energy, with major portion of the rest being in the visible region.
One of the major uses of infrared rays is Infrared photography. Infrared rays are also reflected off objects, just as visible light. Special films or sensors which have the property to 'see in the dark' are used to observe these rays, which enhance different areas according to their heat emission. For e.g., in an infrared photograph, blue sky and water appear nearly black, whereas unexposed skin shows up brightly.
Infrared photography is used to detect pathological tissue growths (thermography) and defects in electronic systems and circuits (due to their increased emission of heat). They can also be used to detect heat leaks in houses and forest fires. Shorter infrared rays are used in remote controls.
Physiotherapists use infrared radiation to warm damaged muscles and so speed up healing. Infrared light can also be sent down optical fibres for cable television and phone links.
Atmospheric haze and certain pollutants that scatter visible light are nearly transparent to parts of the infrared spectrum (scattering efficiency increases with the fourth power of the frequency). Infrared photography of distant objects from the air takes advantage of this phenomenon, to observe cosmic objects through large clouds of interstellar dust. However, since water vapour, O3 and CO2 in the atmosphere absorb large parts of the infrared spectrum, most infrared astronomical observations are carried out at high altitudes, with the help of balloons, rockets and space-crafts.
The infrared absorption and emission characteristics of materials yield important information about the size, shape, and chemical bonding of molecules, atoms and ions present in them. Infrared spectroscopy is a powerful tool for determining the internal structure of molecules and for identifying the amounts of known species in a given sample. Infrared rays emitted by a given substance indicate the difference of some of the internal energy states, which depend on atomic weight and other atomic properties.
Hence, besides for identification, infrared rays can also be used to determine the amount of a known material in a given substance. Infrared spectroscopy is also used to examine archaeological specimens and for detecting forgeries of art and other objects, which, under visible light, resemble the original.
Infrared radiation plays an important role in heat transfer and is integral to the greenhouse effect.
Powerful infrared radiations can be artificially prepared, by using gases like Carbon dioxide and Carbon mono-oxide, and can be used in light radar systems and to modify chemical reactions.
Virtually every object at the Earth's surface emits electromagnetic radiation primarily in the infrared region of the spectrum. Man-made sources of infrared radiation include, besides hot objects, infrared light-emitting diodes (LEDs) and lasers, which are used in some fibre-optic communication systems and light radar systems respectively.
Other applications of infrared light include its use in remote controls, automatic self-focusing cameras, security alarm systems, and night-vision optical instruments.