56 Infrared and Ultraviolet
Herschel was amazed. He guessed that the sun radiated heat waves along with light
waves and that these invisible heat rays refract slightly less while traveling through a prism
than do light rays. Over the course of several weeks, he tested heat rays and found that they
refracted, reflected, bent, etc., exactly like light. Because they appeared below red light,
Herschel named them infrared (meaning below red).
Johann Ritter was born in 1776 in Germany and became a natural science philosopher.
His central beliefs were that there was unity and symmetry in nature and that all natural
forces could be traced back to one prime force, Urkraft.
In 1801, Ritter read about Herschel’s discovery of infrared radiation. Ritter had
worked on sunlight’s effect on chemical reactions and with electrochemistry (the effect of
electrical currents on chemicals and on chemical reactions). During this work he had tested
light’s effect on silver chloride and knew that exposure to light turned this chemical from
white to black. (This discovery later became the basis for photography.)
Ritter decided to duplicate Herschel’s experiment, but to see if all colors darkened silver chloride at the same rate. He coated strips of paper with silver chloride. In a dark room
he repeated Herschel’s set up. But instead of measuring temperature in each color of the
rainbow spectrum projected on wall, Ritter timed how long it took for strips of silver chloride paper to turn black in each color of the spectrum.
He found that red hardly turned the paper at all. He also found that violet darkened paper the fastest.
Again mimicking Herschel’s experiment, Ritter placed a silver chloride strip in the
dark area just beyond the band of violet light. This strip blackened the fastest of all! Even
though this strip was not exposed to visible light, some radiation had acted on the chemicals
to turn them black.
Ritter had discovered radiation beyond violet (ultraviolet) just as Herschel had discovered that radiation existed below the red end of the visible spectrum (infrared).
Fun Facts: A TV remote control uses infrared light to adjust the volume
or change the channel.
More to Explore
Herschel, Frederick. Preliminary Discourse on the Study of Natural Philosophy. Dover, DE: Adamant Media, 2001.
———. Treatise on Astronomy. Dover, DE: Adamant Media, 2001.
Herschel, John. Aspects of the Life and Thought of Sir John Frederick Herschel. New
York: Arno Press, 1994.
Kaufl, Hans. High Resolution Infrared Spectroscopy in Astronomy. New York:
Springer-Verlag, 2003.
Robinson, Michael. Ripples in the Cosmos. Collingdale, PA: Diane Publishing, 2000.