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. Herschel is perhaps most famous for his discovery of the planet Uranus in 1781, the first new planet found since antiquity.
In the year 1800, 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.
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.
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