or in·fra-red



the part of the invisible spectrum that is contiguous to the red end of the visible spectrum and that comprises electromagnetic radiation of wavelengths from 800 nm to 1 mm.


noting or pertaining to the infrared or its component rays: infrared radiation.

Compare ultraviolet.

Origin of infrared

First recorded in 1825–35; infra- + red1 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for infra-red

Contemporary Examples of infra-red

Historical Examples of infra-red

  • There was a little difficulty with the infra-red lamp and goggles.


    William Morrison

  • Captain Greer surveyed the hallways with his infra-red binoculars.

    Anything You Can Do ...

    Gordon Randall Garrett

  • There was enough heat in the cave now to use the infra-red filters.


    Harry Harrison

  • Stanton approached the turn and took off the infra-red goggles.

    Anything You Can Do

    Gordon Randall Garrett

  • The asteroid was a quarter of a mile away, seen through the infra-red.

    The Passing of Ku Sui

    Anthony Gilmore

British Dictionary definitions for infra-red



the part of the electromagnetic spectrum with a longer wavelength than light but a shorter wavelength than radio waves; radiation with wavelength between 0.8 micrometres and 1 millimetre


of, relating to, using, or consisting of radiation lying within the infraredinfrared radiation
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for infra-red

also infrared, 1881 (noun and adjective), from infra- + red.

Online Etymology Dictionary, © 2010 Douglas Harper

infra-red in Medicine




Of or relating to the range of invisible radiation wavelengths from about 750 nanometers, just longer than red in the visible spectrum, to 1 millimeter, on the border of the microwave region.
Generating, using, or sensitive to infrared radiation.


Infrared light or the infrared part of the spectrum.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

infra-red in Science



Relating to the invisible part of the electromagnetic spectrum with wavelengths longer than those of visible red light but shorter than those of microwaves. See more at electromagnetic spectrum.
A Closer Look: In 1800 the astronomer Sir William Herschel discovered infrared light while exploring the relationship between heat and light. Herschel used a prism to split a beam of sunlight into a spectrum and then placed a thermometer in each of the bands of light. When he placed the thermometer just outside the red band, where there was no visible color, the temperature rose, as if light were shining on the thermometer. Further experiment showed that this invisible radiation behaved like visible light in many ways; for example, it could be reflected by a mirror. Infrared radiation is simply electromagnetic radiation with a lower frequency than visible light, having longer wavelengths of 0.7 micrometer to 1 millimeter. Ultraviolet radiation, like infrared radiation, lies just outside the visible part of the spectrum, but with higher frequencies; some animals, such as bees, are capable of seeing such radiation. Both infrared and ultraviolet radiation are often referred to as forms of light, though they cannot be seen by human beings. Heat energy is often transferred in the form of infrared radiation, which is given off from an object as a result of molecular collisions within it. Molecules typically have a characteristic infrared absorption spectrum, and infrared spectroscopy is a common technique for identifying the molecular structure of substances. Astronomers similarly analyze the infrared radiation emitted by celestial bodies to determine their temperature and composition.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.