The Discovery of Infrared by William Hershel

The Discovery of Infrared by William Hershel

Before I started researching this topic I was only aware of Herschel as the discoverer of the planet Uranus in 1781.

It turns out that he made at least one other important discovery, that if infrared light, in 1800.

Herschel's discovery is described at this website in the following way:

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, for each color of the spectrum, placed one bulb in a visible 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 "calorific rays" (derived from the Latin word for 'heat') beyond the red portion of the spectrum. He 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 light, but also because it was the first time that someone showed that there were forms of light that we cannot see with our eyes.

The video below was produced by FLIR Systems, makers of infrared thermal imaging systems. It begins by describing Herschel's experiment and infrared light in general, before describing the use of thermal imaging systems in the modern world.

Over the next century or so the rest of the electromagnetic spectrum was discovered.

The diagram below shows where light and infra red fit into the the electromagnetic spectrum. Infra red has a wavelength between 0.7 and 300 micrometres. A micrometre (micrometer for Americans. The symbol for a micrometre is µm.