In 1800, British astronomer William Herschel, discovered infrared. Because sunlight is composed of a spectrum of various colors and is a source of heat, Herschel wanted to know which color of light is the source of heat. He designed a clever experiment.
He let the sunlight penetrate through a glass prism to generate a spectrum, and then used a thermometer to measure the temperature of each color. Herschel found that the temperature of the spectrum from purple to red will gradually increase, and the region above the red spectrum is the highest part of all spectra.
Because of its thermal radiation, this part of the region cannot be detected by human eyes, and belongs to the invisible light region. Herschel named this invisible radiation "hot rays", and now we call it infrared radiation.