Infrared and Ultraviolet

Energy is radiated by the sun and other stars outside of the narrow visible spectrum of colours.

• Who Discovered it: Frederick Herschel (IR) and Johann Ritter (UV)
• Year of Discovery: 1800 and 1801

 

How was it Discovered?

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 waves 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.

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.

In late 1799 Herschel began a study of solar light. He often used colour filters to isolate parts 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 colours naturally carried more heat than others.

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) to 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 were 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 sun light’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 setup. But instead of measuring temperature in each color of the rainbow spectrum projected on a 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).

Galaxies

Our sun is not the centre of the universe but is rather part of a giant, disc-shaped cluster of stars that floats through space.

• Who discovered it?: Thomas Wright and William Herschel
• Year of Discovery: 1750

How was it discovered?

For thousands of years, scientist believed that the universe consisted of a vast spherical shell of stars, with Earth at its centre. Nothing existed in the immense void between Earth and that shell of stars except the few planets and the sun.

By the mid-1600s, most scientists acknowledged that the sun, not the earth, sat at the centre of the spherical universe. Some prominent scientists (Christian Huygens, for example) believed that stars were really holes in the black sphere of space where light from a luminous region of perpetual day beyond shined through.

Two men’s discoveries combined to establish the existence of dense clusters of stars called galaxies. Born in 1711, English man Thomas Wright taught mathematics and navigation but was a passionate amateur astronomer. As had many astronomers before him, Wright observed that the stars were not evenly spread across the sky. A seeming cloud of faint stars was densely packed along the band called the Milky Way.

This bothered Wright considered that the stars might be spread along the surfaces of a field of giant bubbles. If we were packed along one of those rings of stars, looking along the ring would cause us to see more stars than if we looked straight out from it. He then considered the rings of Saturn and proposed that the stars might be packed into wide rings or a thin disk. If we were on that disc, it would account for the uneven distribution of stars we saw—even if the stars were really evenly spaced across that disk.

In 1750 Wright published a book, An Original Theory on New Hypothesis of the Universe, in which he proposed this theory. He was the first to use the word galaxy to describe a giant cluster of stars.

Five years later, famed astronomer and mathematician Immanuel Kant
proposed a similar arrangement of the stars into a giant disk-shaped cluster.
English astronomer William Hershel (born in 1738) read with interest Wright’s theory.

In 1785 Herschel decided to use statistical methods to count the stars. He surely couldn’t count them all. So he randomly picked 683 small regions of the sky and set about counting the stars in each region using a 48-inch telescope—considered a giant scope at the time.

Herschel quickly realized that the number of stars per unit area of sky rose steadily as he approached the Milky Way and spiked in regions in the Milky Way. (The number of stars per unit area of sky reached a minimum in directions at right angles to the Milky Way.) This made Herschel think of Wright’s and Kant’s theories. Hershel concluded that his counting results could only be explained if most of the stars were compacted into a lens-shaped mass and that the sun was buried in this lens. Herschel was the first to add statistical measurement to Wright’s discovery of the existence and shape of galaxies.