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William Herschel, an amateur astronomer famous for the discovery of Uranus in 1781, made an important discovery in 1800.
Herschel was familiar with Newton’s discovery that sunlight could be separated into that separate chromatic components via refraction
through a glass prism. Herschel thought that the colors themselves might contain different levels of heat, so he devised a clever experiment
to investigate this. Herschel passed sunlight through a glass prism to create a spectrum (the rainbow created when light is divided into its color
components) and measured the temperatures of the different colors. He used three thermometers with blackened bulbs and placed one bulb in each color
while the other two were placed outside the spectrum as controls.
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Sunlight passes through a prism forming the usual rainbow spectrum. A row of thermometers is positioned
on a table beyond the red end of the spectrum. Thermometer 1, aligned with the spectrum, registers a
rise in temperature, while the control thermometers 2 and 3 do not. |
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| As he measured the temperature of the violet, blue, green, yellow, orange and red light, he noticed that all the colors had
temperatures higher than the controls and that the temperature increased from the violet to the red part of the spectrum. After understanding this pattern, Herschel measured the temperature
just beyond the red portion of the spectrum and found this area had the highest temperature of all and thus contained the most heat. What Herschel discovered was a form of light beyond red
light. Herschel’s experiment was important not only because it lead to the discovery of infrared light, but because it was the first experiment that showed there were forms of light not visible
to the human eye.
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| Beginning with Herschel’s observations we now understand the full nature
of electromagnetic radiation and have developed a wide range of technologies to observe it and exploit it to man’s benefit. |
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