Infrared radiation is electromagnetic radiation having a wavelength in the range from circa 75 x 10⁻⁶ cm to circa 100,000 x 10⁻⁶ cm (0.000075-0.1 cm). Infrared rays thus occupy that part of the electromagnetic spectrum with a frequency less than that of visible light and greater than that of most radio waves, although there is some overlap. Infrared radiation is thermal, or heat, radiation. Its applications are many, from radiometers in heat-seeking missiles and cameras, heat lamps and thermal imaging in the medical field, to infrared astronomy.
The German-born British astronomer Frederick William Herschel discovered infrared radiation in 1800 after discovering that the colours which formed after dispersing sunlight generated different amounts of heat. He then created an experimental setup using a prism to disperse sunlight onto already-installed thermometers in order to conclusively confirm his findings. He found (incorrectly as he used a prism) that the red portion of the spectrum produced more heat in the thermometer than the bluer portion on the same spectrum. More importantly, he discovered that if the thermometer was placed beyond the red portion where there was no colour, the temperature would be even higher. Here is his original drawing of the setup just described; though over here the bulbs of the thermometers are placed in the infrared region of the spectrum:
About the experiment, he notes in his publication that, “A beam of radiant heat, emanating from the sun, consists of rays that are differently refrangible. The range of their extent, when dispersed by a prism, begins at violet coloured light, where they are most refracted, and have the least efficacy. We have traced these calorific rays throughout the whole extent of the prismatic spectrum; and found their power increasing, while their refrangibility was lessened, as far as to the confines of red-coloured light. But their diminishing refrangibility, and increasing power, did not stop here; for we have pursued them a considerable way beyond the prismatic spectrum, into an invisible state, still exerting their increasing energy,with a decrease of refrangibility up to the maximum of their power; and have also traced them to that state where, though still less refracted, their energy, on account, we may suppose, of their now failing density, decreased pretty fast; after which, the invisible thermometrical spectrum, if I may so call it, soon vanished.” The “thermometrical spectrum” he refers to here is the infrared region.
Here is a portrait of William Herschel:
 https://books.google.ae/books?id=TIw5AQAAMAAJ&source=gbs_navlinks_s, pages 255-273, 291-292 and plate XI.