It is extremely noticeable when police sirens or ambulance emergency vans rise in pitch as they approach us, and then eventually the pitch lowers down when they go off away from us into the distance. This is referred to as Doppler effect. More recently, the Doppler effect took centre-stage as investigators used it to trace the Malaysian aircraft Flight 370's disappearance.
What is the Doppler Effect?
In physics parlance as our professors taught us, or as Oxford Dictionary defines it, Doppler effect is "the apparent change in the observed frequency of a wave as a result of relative motion between the source and the observer." Earlier, I mentioned about the police and ambulance sirens pitches that we loudly hear the nearer they get to us.
In like manner, the sound made by a low-flying aircraft as it approaches appears to fall in pitch as it passes and flies away. Actually, the frequency of the aircraft engine remains constant but as it is approaching more sound waves (per second) impinge on our ear.
The Scientist Behind It: Christian Doppler
Christian Johann (Andreas) Doppler (29 November 1803-17 March 1853), was an Austrian physicist and mathematician. He was born in Salzburg and died in Venice Italy at the age of 49. He postulated his principle, later coined the Doppler Effect, that the observed frequency of a wave depends on the relative speed of the source and the observer. He tried to use this concept for explaining the colour of binary stars.
Applications of Doppler Effect: On Sound and Light Waves
Sound Waves. Doppler explained the effect pointing out that when the source of sound is moving towards the observer, sound waves reach the ear at shorter intervals, therefore the higher pitch. It follows that as the source is moving away, the waves reach the ear at longer intervals, therefore the lower pitch. Doppler effect also occurs when the source of sound is stationary but the observer is moving.
Light Waves. In like manner, Doppler also predicted that the effect would apply to light waves, however he provided no explanations. In 1848, Armand Fizeau, a French physicist, showed that the Doppler effect applies to light coming from distant stars. This was the first successful experiment to determine the speed of light.
Other Claimants of the Doppler Effect
It is also worth noting that according to Heriot-Watt University/Dept of Mathematics, John Russell Scott (1808-1882), made one of the first experimental observations of Doppler effect, and gave an independent explanation of the theory. This is generally not known, as the Doppler effect is commonly attributed as first described by the Austrian physicist Christian Doppler, with the discovery named after him. Scott Russell's short reprinted report gives no date for his original observation.
The credit for first published account of this effect belongs to the Dutch meteorologist Buys Ballot in 1845 who tested the Doppler effect for sound waves by using an orchestra of trumpeteers standing in an open car of a railroad train, yet demonstrating the correctness of the effect.
A Brief and Simple Summary
Simply summarised, the Doppler effect, named after Christian Doppler, is any source of sound or light that moves away from an observer changes in frequency with reference to the observer. The frequency is decreased if either the source or the observer is moving away from the other.
Resources:
- Dictionary of Physics. (2005) Oxford University Press, 5th Edition. London.
- "How Investigators of flight Malaysia MH370's pinpointed its last location using 19th century physics." Mirror.Co.UK. Accessed April 2, 2014.
- "How Investigators Used the Doppler Effect to Trace Flight 370". The Wall Street Journal - SoutheastAsia. Accessed April 2, 2014.
- "MH370 Search: How New Satellite Data Confirmed Malaysia Airlines Plane was Lost. Sydney Morning Herald. Accessed April 2, 2014.
- Russell, John Scott. "On Certain Effects produced on Sound by the Rapid Motion of the Observer." Heriot-Watt University / Dept of Mathematics. Accessed April 2, 2014.
- Sciencia: mathematics, Physics, Chemistry, Biology, and Astronomy for All. (2011). Edited by John Martineau. Bloomsbury Wooden Books, U.S. Edition.
Image Credit:
Doppler Effect Diagram. Waves emitted by a source moving from the right to the left. The frequency is higher on the left (ahead of the source) than on the right. en.wikipedia.org, by Tkarcher, improved by Tatoute. Accessed April 2, 2014
Note: I originally published this piece at my former blog InventionsDiscoveries.com, 2 April 2014.
(c) 2016 Tel Asiado, written for InspiredPenWeb.com. All Rights Reserved.
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