The abbreviation IR refers to infrared, a term that literally means “below red.” The Latin prefix infra- means “below” or “beneath,” and infrared lies beyond the red end of the visible-light spectrum.
Infrared radiation has wavelengths longer than visible red light and therefore lower frequencies and lower photon energies. It is invisible to human eyes, but we commonly experience it as radiant heat from warm objects, sunlight, heaters, and living bodies. Depending on the convention, infrared is often described as spanning roughly 700 nanometres to 1 millimetre.
The phenomenon was discovered by William Herschel in 1800. While separating sunlight with a prism, he moved a thermometer through the colored spectrum and found that the temperature rose most strongly just beyond the visible red region, where no light could be seen. This revealed an invisible form of radiation and inspired the name infrared.
A common mix-up is confusing “below” with physically underneath. The word refers to infrared’s position beyond red in a spectrum, not to a beam traveling downward. Infrared is also not the same as ultraviolet: ultraviolet lies beyond violet, has shorter wavelengths, and generally carries more energy.