Frederick William Herschel (1738-1822)was born in Hanover, Germany
and became well known as both a musician and as an astronomer. He
moved 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 famous for his
discovery of the planet Uranus in 1781, the first new planet found
Herschel made another dramatic discovery in 1800. He wanted to know
how much heat was passed through the different colored filters he
used to observe sunlight. He had noted that filters of different
colors seemed to pass different amounts of heat. Herschel thought
that the colors themselves might be of varying temperatures and
so he devised a clever experiment to investigate his hypothesis.
He directed sunlight through a glass prism to create a spectrum
the rainbow created when light is divided into its colors
and then measured the temperature of each color. Herschel
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 individual temperatures of
the violet, blue, green, yellow, orange and red light, he noticed
that all of the colors had temperatures higher than the controls.
Moreover, he found that the temperatures 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 additional 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 in a manner similar to visible light. What Sir William
had discovered was a form of light (or radiation) beyond red light,
now known as infrared radiation. [The prefix infra means below.]
Herschel's experiment was important because it marked the first
time that someone demonstrated that there were types of light that
we cannot see with our eyes.
Herschel's Experiment continues in PDF form:
more about Herschel and his experiments on the Space
Infrared Telescope Facility's Education and Outreach Home Page.
View the Space
Infrared Telescope Facility's Education and Outreach Home Page
in PDF form.
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