Figure: Infrared light

This illustration shows the different kinds of light, or so-called electromagnetic radiation. The "size" of the light waves - the wavelength - increases towards the right. The energy of the light increases towards the left, meaning that the shorter the wavelength of the light, the higher the energy it has. The wavelengths at which two famous infrared projects, ISO and NICMOS, observed are also shown in the figure, as well as the two future missions, Herschel and NGST.

All types of light are "electromagnetic radiation". Electromagnetic radiation propagates through space as a wave. Although this wave always travels at the same speed in a vacuum (the famous speed of light, 300 000 kilometres per second), some of its characteristics vary depending on its energy. Ultraviolet radiation, for instance, is more energetic than visible light, which in turn is more energetic than infrared radiation. Astronomers, for natural reasons, first studied the sky using visible light, but as science and technology have become more sophisticated, radiation from other parts of the electromagnetic spectrum has also come into play. Infrared radiation is the most recent to be exploited, thanks to a combination of new developments in detector technology and the advent of space-based telescopes.

This illustration relates the characteristic length (the wavelength) of the light waves, to the energy of the individual light particles or photons. Notice that visible light has a wavelength of a few tenths of a micron (a micron (μm) is a thousandth of a millimetre) and occupies a remarkably small part of the whole electromagnetic spectrum. Infrared light lies between the red end of the visible and the microwave part of the electromagnetic spectrum. It can have wavelengths ranging from just less than 1 micron (near-infrared) to about 1 mm (sub-millimetre waves). In daily life, infrared radiation has numerous uses: in remote controls, police speed measurement systems, night-vision binoculars, security systems, auto-focus cameras, and car door locks, to name but a few.

Credit: "The infrared revolution, unveiling the hidden universe", Monica Salomone, Lars Lindberg Christensen, Bruce Battrick, Martin Kornmesser (Hubble ESA Information Centre)

http://www.spacetelescope.org/about/further_information/brochures/html/infrared.html