Within the electromagnetic spectrum, infrared waves occur at frequencies above those of microwaves and just below those of red visible light, hence the name "infrared."

Room temperature objects (about 300 kelvins [K]) emit radiation with a peak intensity in the far infrared; radiation from toaster filaments and light bulb filaments (about 700 K and 2,000 K, respectively) also peak in the infrared, though their spectra extend progressively into the visible; the 6,000 K surface of the Sun emits blackbody radiation that peaks in the centre of the visible range.

Sir William Herschel first discovered infrared light in 1800. He split light into a rainbow (called a spectrum) by passing sunlight through a prism, and then placed a thermometer in different colors in that spectrum.

Infrared radiation was discovered by William Herschel in 1800. He was studying the heating effect of different colours of light by using a prism to produce a spectrum of colours and thermometers to measure their heating effect. He noticed that the heating effect got stronger as he went from the blue end of the spectrum to the red.