Science & Technology

How were Infrared and Ultraviolet Rays Discovered?

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In the years 1800 and 1801, Frederick Herschel and Johann Ritter discovered the infrared (IR) and ultraviolet (UV) rays, says Prof. Ashoka. An exclusive for Different Truths.  

Energy is radiated by the sun and other stars outside of the narrow visible spectrum of colours. This was discovered by Frederick Herschel (IR) and Johann Ritter (UV).

Infrared and ultraviolet radiation are key parts of our scientific development over the past 200 years. Yet until 1800 it never occurred to anyone that radiation could exist outside the narrow band that human eyes detect. The discovery of infrared and ultraviolet light expanded science’s view beyond the visible light to the whole radiation spectrum, from radio wave to gamma rays.

Infrared (IR) radiation has been key to many astronomical discoveries. In addition, earth science uses IR to measure heat in studies of everything from ocean temperatures to forest health. IR sensors power burglar alarms, fire alarms, and police and fire infrared de­tectors. Biologists have discovered that many birds and insects detect IR radiation with their eyes Ultraviolet light (UV) led to a better understanding of solar radiation and to high-energy parts of the spectrum, including X-rays, microwaves, and gamma rays.

How Was It Discovered?

Frederick Herschel was born in Hanover, Germany, in 1738. As a young man, he grew into a gifted musician and astronomer. It was Herschel who discovered the planet Uranus in 1781, the first new planet discovered in almost 2,000 years.

It was Herschel who discovered the planet Uranus in 1781, the first new planet discovered in almost 2,000 years.

In late 1799 Herschel began a study of solar light. He often used color filters to isolate part of the light spectrum for these studies and noted that some filters grew hotter than oth­ers. Curious about this heat in solar radiation, Herschel wondered if some colors naturally carried more heat than others.

To test this idea, Herschel built a large prism.

To test this idea, Herschel built a large prism. In a darkened room, he projected the prism’s rainbow light spectrum onto the far wall and carefully measured the temperature in­side each of these separate coloured light beams.

Herschel was surprised to find that the temperature rose steadily from violet (coolest) a maximum in the band of red light.

Herschel was surprised to find that the temperature rose steadily from violet (coolest) a maximum in the band of red light. On a sudden impulse, Herschel placed a thermometer in the dark space right next to the band of red light (just beyond the light spectrum).

This thermometer should have stayed cool. It was not in any direct light. But it didn’t. This thermometer registered the most heat of all.

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 the 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

Johann Ritter was born in 1776 in Germany and became a natural science

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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 colours darkened sil­ver chloride at the same rate.

Ritter decided to duplicate Herschel’s experiment but to see if all colours 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 the 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).

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.


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