On 11th February 1800 astronomer Frederick William Herschel discovered the infrared light using a glass prism and a thermometer, which drew him to the conclusion that there was a form of invisible light beyond the visible spectrum.
Sir Frederick William Herschel (1738-1822) was born in Hanover, Germany but emigrated to England in 1757. Together with his sister Caroline he devoted himself to astronomy, and both created a lot of telescopes. The discovery he is well-known for is the planet Uranus in 1781. But Herschel and his sister discovered constellations and comets.
In the year 1800, Herschel made a unique discovery while trying to find out how much heat went through different colour filters. With these filters he peered at the sun and he saw that there were differences between the heat levels that let pass these filters according to the measured colour. From there, he designed an experiment to test his hypothesis: the colours themselves could contain different levels of heat.
The experiment consisted of directing a ray of light to pass through a glass prism. The aim was to create a spectrum (a Rainbow), whose forms appear when the light is separated into colours. Herschel measured the temperature of each colour using three thermometers with the blackened thermometer bulb to improve heat absorption. He put the thermometer bulb into each colour and also beyond the spectrum, so that it could obtain control samples. While he was measuring the temperatures of the violet, blue, green, yellow, orange and red light he realized that all the colours had higher temperatures than the average obtained in the controls. The temperature of the colours increased progressively from the violet colour to the red colour part of the spectrum.
Herschel decided to measure the temperature beyond the red colour, a region not touched by the red light colour. There he found, surprisingly, that this region had the highest temperature. These light-rays are called “calorific”, which are beyond the red part of the spectrum (infra red or IR). At the end of his experiments he discovered that these kind of rays (IR) reflected, refrected, absorbed, and passed the same way as the visible light.
This light “beyond the red light” was called infrared rays or infrared radiation (IR), below the red light. This experiment showed that there were forms of light that we cannot see with our eyes.
The temperature measuring experiment of the different colours is explained very well. You can do it yourself following the instructions of La Ciencia de Spitzer, a Science Education Web.
Main picture from: http://elsofista.blogspot.com/2010/10/el-descubrimiento-del-infrarrojo.html