Sir Frederick William Herschel (1738-1822) was born in Hanover, Germany, and became well known as both a musician and an astronomer. He emigrated to England in 1757, and with his sister Caroline, constructed telescopes to survey the night sky. Their work resulted in several catalogs of double stars and nebulae. William Herschel is perhaps most famous for his discovery of the planet Uranus in 1781, the first new planet found since antiquity. Caroline Herschel gained fame for the discovery of several comets. In the year 1800, William Herschel made another very important discovery. He was interested in learning how much heat passed through the different colored filters he used to observe the Sun and noticed that filters of different colors seemed to pass different levels of heat. Herschel thought that the colors themselves might contain different levels of heat, so he devised a clever experiment to investigate his hypothesis.

Herschel directed sunlight through a glass prism to create a spectrum - the "rainbow" created when light is divided into its colors - and measured the temperature of each color. He used three thermometers with blackened bulbs (to better absorb the heat) and placed one bulb in each color while the other two were placed beyond the spectrum as control samples. As he measured the temperatures of the violet, blue, green, yellow, orange and red light, he noticed that all of the colors had temperatures higher than the controls and that the temperature of the colors increased from the violet to the red part of the spectrum. After noticing this pattern, Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region apparently devoid of sunlight. To his surprise, he found that this region had the highest temperature of all.

Herschel performed further experiments on what he called the "calorific rays" that existed beyond the red part of the spectrum and found that they were reflected, refracted, absorbed and transmitted just like visible light. What Sir William had discovered was a form of light (or radiation) beyond red light. These "calorific rays" were later renamed infrared rays or infrared radiation (the prefix infra means `below'). Herschel's experiment was important not only because it led to the discovery of infrared, but also because it was the first time that someone showed that there were forms of light that we cannot see with our eyes. Herschel's original prism and mirror are on display at the National Museum of Science and Industry in London, England.

Infrared image of a hurricane (NASA)

Today, infrared technology has many exciting and useful applications. In the field of infrared astronomy, new and fascinating discoveries are being made about the Universe. Medical infrared imaging is a very useful diagnostic tool. Infrared cameras are used for police and security work as well as in fire fighting and in the military. Infrared imaging is used to detect heat loss in buildings and in testing electronic systems. Infrared satellites have been used to monitor the Earth's weather, to study vegetation patterns, and to study geology and ocean temperatures.

If you are interested in trying this experiment see our activity on the Herschel Infrared Experiment.

The year 2000 marked the 200th aniversary of Sir William Herschel's discovery of infrared.