In William Herschel’s 1800 prism experiment, what invisible radiation was detected beyond red light?

William Herschel’s 1800 prism experiment detected infrared radiation beyond the red end of visible light.

Herschel used a glass prism to spread sunlight into a spectrum and placed thermometers in different coloured regions. He found that the thermometer just beyond the visible red region became warmer than those illuminated by the visible colours. That result showed that sunlight contained invisible radiation with a heating effect.

Herschel called it “calorific rays,” but it is now known as infrared radiation. Infrared has longer wavelengths than visible red light, although its exact boundary with visible light is gradual rather than sharply defined.

The experiment is often confused with the later discovery of ultraviolet radiation. Johann Wilhelm Ritter detected ultraviolet effects beyond violet in 1801. Herschel’s work therefore revealed the invisible, longer-wavelength side of the spectrum, while Ritter’s work explored the shorter-wavelength side.