William Herschel detects infrared light while measuring the temperatures in the colors of the spectrum created by passing sunlight through a prism. Herschel noticed that the temperature increased from the blue to the red part of the spectrum. He then measures the temperature in the region just past the red where there is no visible light, and to his surprise, this region registers the highest temperature of all. This experiment shows for the first time that there is light that we cannot see with our eyes. Infrared becomes the first form of invisible radiation to be discovered. Herschel's experiment also shows that the Sun is a source of infrared light.