infrared

Infrared (IR or infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light and shorter than that of radio waves. Cited wavelength-ranges vary somewhat: a representative example is 750 nm to 1 mm, corresponding to EMR frequencies between 300 GHz and 400 THz. Portions of its wavelength-span are blocked by Earth atmosphere and infrared bands defined for astronomical use are aligned with the intervening atmospheric windows and identified by the same alphabetic codes:

(Note that infrared bands are also defined for infrared-based communications, similarly labeled with letters, but they do not correspond to these astronomy-related bands.) Some near-infrared observations are made through visible-light telescopes, whose mirrors are suitable for adjacent portions of the EMR spectrum. The shortest-wavelength infrared telescopes (submillimeter) are more like radio telescopes and similar interferometry is of use. IR is often used to detect temperature at a distance, because virtually anything with a temperature less than ~3800 K emits black-body radiation peaking in infrared (thermal IR) and the temperature can be determined (in principle) by Wien's displacement law. IR is also used for sensing "through walls" because some material that is opaque to visible light is transparent to IR. IR sensors are used in the study of planets from space probes: e.g., IR cameras or spectrographs. Infrared luminosity (LIR or L_IR), an object's luminosity in the infrared is of interest, often indicating intervening obscuring gas and dust, heated so as to produce an infrared thermal radiation.

Infrared astronomy can view virtually anything producing thermal radiation, but is especially useful for objects whose EMR arrives within the infrared range. Prominent are M-type stars which are dim outside the infrared, and many objects at cosmological distances, much of whose EMR is redshifted into the infrared, motivating efforts such as the James Webb Space Telescope (JWST).