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?


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.

The Existence of Molecules

“A molecule is a group of attached atoms. An atom uniquely identifies one of the 100+ chemical elements that make up our planet. Bonding a number of different atoms together makes a molecule, which uniquely identifies one of the many thousands of substances that can exist.”

  • Who Discovered it?: Amedeo Avogadro
  • Year of the Discovery: 1811

How was it Discovered?

2010-11-08-16-23-00-1-amadeo-avogadro-is-noted-for-his-contributions-toIf atoms are the basic building block of each element, then molecules are the basic building blocks of each substance on Earth.

Scientists were stalled by their inability to accurately imagine—let alone detect—particles as small as an atom or a molecule. Many had theorized that some tiny particle (that they called an atom) was the smallest possible particle and the basic unit of each element.

However, the substances around us were not made of individual elements. Scientists were at a loss to explain the basic nature of substances.

In the spring of 1811, 35-year-old college professor Amedeo Avogadro sat in his classroom scowling at two scientific papers laid out on his desk. Avogadro taught natural science classes at Vercelli College in the Italian mountain town of Turin. Twenty-five students sat each day and listened to Professor Avogadro lecture, discuss, and quiz them on whatever aspects of science caught his fancy. This day he read these two papers to his class, claimed that he saw an important mystery in them, and challenged his students to find it.

In the two papers, the English chemist, Dalton, and the French chemist, Gay-Lussaceach described an experiment in which they combined hydrogen and oxygen atoms to create water. Both reported that it took exactly two liters of gaseous hydrogen atoms to combine with exactly one liter of oxygen atoms to produce exactly two liters of gaseous water vapor. Dalton claimed that this experiment proved that water is the combination of two atoms of hydrogen and one atom of oxygen. Gay-Lussac also claimed it proved that a liter of any gas had to contain exactly the same number of atoms as a liter of any other gas, no matter what gas it was.

These studies were heralded as major breakthroughs for chemical study. But from his first reading, Professor Avogadro was bothered by a nagging contradiction. Both Dalton and Gay-Lussac started with exactly two liters of hydrogen and one liter of oxygen. That’s a total of three liters of gas. But they both ended with only two liters of water vapor gas. If every liter of every gas has to have exactly the same number of atoms, then how could all the atoms from three liters of gas fit into just two liters of water vapor gas?

The Turin cathedral bell chimed midnight before the answer struck Avogadro’s mind. Dalton and Gay-Lussac had used the wrong word. What if they had each substituted “a group of attached atoms” for atom?

Avogadro created the word molecule (a Greek word meaning, “to move about freely in a gas”) for this “group of attached atoms.” Then he scratched out equations on paper until he found a way to account for all of the atoms and molecules in Dalton’s and Gay-Lussac’s experiments.

If each molecule of hydrogen contained two atoms of hydrogen, and each molecule of oxygen contained two atoms of oxygen, then—if each molecule of water vapor contained two atoms of hydrogen and one atom of oxygen, as both scientists reported each liter of hydrogen and each liter of oxygen would have exactly the same number of molecules as each of the two resulting liters of water vapor (even though they contained a different number of atoms)!

And so it was that, without ever touching a test tube or chemical experiment of any kind, without even a background in chemistry, Amedeo Avogadro discovered the existence of molecules and created the basic gas law—every liter of a gas contains the same number of molecules of gas.

Avogadro’s discovery (and the related Avogadro’s Number) have become one of the cornerstones of organic and inorganic chemistry as well as the basis for the gas laws and much of the development of quantitative chemistry.

Fun Facts: The small est molecule is the hydrogen molecule—just two protons and two electrons. DNA is the largest known naturally occurring molecule, with over four billion atoms—each containing a number of protons, neutrons, and electrons.

Electrochemical Bonding

Molecular bonds between chemical elements are electrical in nature

  • Who Discovered it?: Humphry Davy.
  • Year of Discovery: 1806

How was it Discovered?

Davy discovered that the chemical bonds between individual atoms in a molecule are electrical in nature. We now know that chemical bonds are created by the sharing or transfer of electrically charged particles—electrons—between atoms. In 1800, the idea that chemistry somehow involved electricity was a radical discovery.250px-Sir_Humphry_Davy,_Bt_by_Thomas_Phillips

Humphry Davy was born in 1778 along the rugged coast of Cornwall, England. He received only minimal schooling and was mostly self-taught. As a young teenager, he was apprenticed to a surgeon and apothecary. But the early writings of famed French scientist Antoine Lavoisier sparked his interest in science.

In 1798 Davy was offered a chance by wealthy amateur chemist Thomas Beddoes to work in Bristol, England, at a new lab Beddoes built and funded. Davy was free to pursue chemistry-related science whims. He experimented with gases in 1799, thinking that the best way to test these colorless creations was to breathe them. He sniffed nitrous oxide (N2O) and passed out, remembering nothing but feeling happy and powerful. After he reported its effect, the gas quickly became a popular party drug under the name “laughing gas.” Davy used nitrous oxide for a wisdom tooth extraction and felt no pain.

Even though he reported this in an article, it was another 45 years before the medical profession finally used nitrous oxide as its first anesthetic.

Davy also experimented with carbon dioxide. He breathed it and almost died from carbon dioxide poisoning. A born showman, movie-star handsome, and always fashionably dressed, Davy delighted in staging grand demonstrations of each experiment and discovery for thrilled audiences of public admirers.

In 1799, Italian Alessandro Volta invented the battery and created the world’s first manmade electrical current. By 1803, Davy had talked Beddoes into building a giant “Voltaic Pile” (battery) with 110 double plates to provide more power. Davy turned his full attention to experimenting with batteries. He tried different metals and even charcoal for the two electrodes in his battery and experimented with different liquids (water, acids, etc.) for the liquid (called an electrolyte) that filled the space around the battery’s plates.

In 1805 Davy noticed that a zinc electrode oxidized while the battery was connected.  That was a chemical reaction taking place in the presence of an electrical current. Then he noticed other chemical reactions taking place on other electrodes. Davy realized that the battery (electric current) was causing chemical reactions to happen.

As he experimented with other electrodes, Davy began to realize the electrical nature of chemical reactions. He tried a wide variety of materials for the two electrodes and different liquids for the electrolyte.

In a grand demonstration in 1806, Davy passed a strong electric current through pure water and showed that he produced only two gasses—hydrogen and oxygen. Water molecules had been torn apart by an electric current. This demonstration showed that an electrical force could tear apart chemical bonds. To Davy, this meant that the original chemical bonds had to be electrical in nature or an electric current couldn’t have ripped them apart.

Davy had discovered the basic nature of chemical bonding. Chemical bonds were somehow electrical. This discovery radically changed the way scientists viewed the formation of molecules and chemical bonds.

Davy continued experiments, passing electrical currents from electrode to electrode through almost every material he could find. In 1807 he tried the power of a new battery with 250 zinc and copper plates on caustic potash and isolated a new element that burst into brilliant flame as soon as it was formed on an electrode. He named this newly discovered element; PotassiumA month later he isolated Sodium. Davy had used his grand discovery to discover two new elements.

Davy’s discovery started the modern field of electrochemistry and redefined science’s view of chemical reactions and how chemicals bond together. Not to mention that he discovered the two new very important elements, Potassium, and Sodium.

Fun Facts: A popular use of electrochemical bonding is in cookware. The process unites the anodized surface with the aluminum base, creating a nonporous surface that is 400 percent harder than aluminum.