Doppler Effect


“Sound- and light-wave frequencies shift higher or lower depending on whether the source is moving toward or away from the observer.”

  • Who Discovered it?: Christian Doppler
  • Year of the Discovery: 1848

How was it Discovered?

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Austrian-born Christian Doppler was a struggling mathematics teacher—struggling both because he was too hard on his students and earned the wrath of parents and administrators and because he wanted to fully understand the geometry and mathematical concepts he taught. He drifted in and out of teaching positions through the 1820s and 1830s as he passed through his twenties and thirties. Doppler was lucky to land a math teaching slot at Vienna Polytechnic Institute in 1838.

By the late 1830s, trains capable of speeds in excess of 30 mph were dashing across the countryside. These trains made a sound phenomenon noticeable for the first time. Never before had humans traveled faster than the slow trot of a horse. Trains allowed people to notice the effect of an object’s movement on the sounds that the object produced.

Doppler intently watched trains pass and began to theorize about what caused the sound shifts he observed. By 1843 Doppler had expanded his ideas to include light waves and developed a general theory that claimed that an object’s movement either increased or decreased the frequency of sound and light it produced as measured by a stationary observer. Doppler claimed that this shift could explain the red and blue tinge to the light of distant twin stars. (The twin circling toward Earth would have its light shifted to a higher frequency—toward blue. The other, circling away, would shift lower, toward red.)

In a paper he presented to the Bohemian Scientific Society in 1844, Doppler presented his theory that the motion of objects moving toward an observer compresses sound and light waves so that they appear to shift to a higher tone and to a higher frequency color (blue). The reverse happened if the object was moving away (a shift toward red). He claimed that this explained the often observed red and blue tinge of many distant stars’ light. Actually, he was wrong. While technically correct, this shift would be too small for the instruments of his day to detect.

Doppler was challenged to prove his theory. He could n’t with light because telescopes and measuring equipment were not sophisticated enough. He decided to demonstrate his principle with sound.

In his famed 1845 experiment, he placed musicians on a railway train playing a single note on their trumpets. Other musicians, chosen for their perfect pitch, stood on the station platform and wrote down what note they heard as the train approached and then receded. What the listeners wrote down was consistently first slightly higher and then slightly lower than what the moving musicians actually played.

Doppler repeated the experiment with a second group of trumpet players on the station platform. They and the moving musicians played the same note as the train passed. Listeners could clearly hear that the notes sounded different. The moving and stationary notes seemed to interfere with each other, setting up a pulsing beat.

Having proved the existence of his effect, Doppler named it the Doppler Shift. However, he never enjoyed the fame he sought. He died in 1853 just as the scientific community was beginning to accept and to see the value of, his discovery.

The Doppler Effect is one of the most powerful and important concepts ever discovered for astronomy. This discovery allowed scientists to measure the speed and direction of stars and galaxies many millions of light years away. It unlocked mysteries of distant galaxies and stars and led to the discovery of dark matter and of the actual age and motion of the universe. Doppler’s discovery has been used in the research efforts of a dozen scientific fields.

Few single concepts have ever proved more useful. Doppler’s discovery is considered to be so fundamental to science that it is included in virtually all middle and high school basic science courses.

 

 

Fun Facts: Doppler shifts have been used to prove that the universe is expanding. A convenient analogy for the expansion of the universe is a loaf of unbaked raisin bread. The raisins are at rest relative to one another in the dough before it is placed in the oven. As the bread rises, it also expands, making the space between the raisins increase. If the raisins could see, they would observe that all the other raisins were moving away from them although they themselves seemed to be stationary within the loaf. Only the dough—their “universe”—is expanding.

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).