Date: 5/19/96 at 22:3:19 From: Anonymous Subject: Doppler Effect Dear Dr. Math, My friend is doing a project on the Doppler Effect and is having incredible trouble getting any understandable research. If you could send me any information on this topic, it would be greatly appreciated. Thanks in advance. Rob
Date: 5/19/96 at 22:37:58 From: Doctor Sarah Subject: Re: Doppler Effect A search of the Web using AltaVista ( http://altavista.digital.com ) yields a number of sites on the Doppler Effect. Here's one explanation: http://www.discovery.com/DCO/doc/1012/world/science/planethunters/planethunt1.2.html This fundamental law of physics, discovered by 19th century Austrian scientist Christian Doppler, applies to sound and radar, as well as to light. When a sound moves toward a listener, sound waves shorten; when itmoves away, they lengthen. As with the familiar example of an approaching car horn we hear this as a change in pitch. As the car moves closer, the horn's successive sound waves reach us more quickly and therefore with higher pitches. As the car speeds away, the sound waves are dragged out and the horn drops in pitch. Similarly, when a star moves toward Earth, its light waves shorten and shift toward the ultraviolet end of the color spectrum. When it moves away, the waves lengthen and shift toward the ultrared end. This "wobble" back and forth reveals the star's speed of motion and may indicate gravitational pull from a nearby planet. At http://hurlbut.jhuapl.edu/NEAR/Education/lessonDoppler/lpdop2.html there are activities for experimenting to produce and explain the Doppler effect using a vacuum cleaner hose and a slinky. Here's a bit from http://126.96.36.199/Cyberia/Bima/doppler.html : The Doppler Effect A Familiar Example Heard an ambulance go by recently? Remember how the siren's pitch changed as the vehicle raced towards, then away from you? First the pitch became higher, then lower. Originally discovered by the Austrian mathematician and physicist, Christian Doppler (1803-53), this change in pitch results from a shift in the frequency of the sound waves, as illustrated in the following picture. [image of red and blue shift] As the ambulance approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch. As the ambulance recedes, the sound waves are stretched relative to the observer, causing the siren's pitch to decrease. By the change in pitch of the siren, you can determine if the ambulance is coming nearer or speeding away. If you could measure the rate of change of pitch, you could also estimate the ambulance's speed. By analogy, the electromagnetic radiation emitted by a moving object also exhibits the Doppler effect. The radiation emitted by an object moving toward an observer is squeezed; its frequency appears to increase and is therefore said to be blueshifted. In contrast, the radiation emitted by an object moving away is stretched or redshifted. As in the ambulance analogy, blueshifts and redshifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer. The Doppler Effect In Astronomy In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. Radiation is redshifted when its wavelength increases, and is blueshifted when its wavelength decreases. Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth. For example the spectral lines emitted by hydrogen gas in distant galaxies is often observed to be considerably redshifted. The spectral line emission, normally found at a wavelength of 21 centimeters on Earth, might be observed at 21.1 centimeters instead. This 0.1 centimeter redshift would indicate that the gas is moving away from Earth at over 1,400 kilometers per second (over 880 miles per second). Shifts in frequency result not only from relative motion. Two other phenomena can substantially the frequency of electromagnetic radiation, as observed. One is associated with very strong gravitational fields and is therefore known as Gravitational Redshift . The other, called the Cosmological Redshift, results not from motion through space, but rather from the expansion of space following the Big Bang, the fireball of creation in which most scientists believe the universe was born. ___________________________ There's a funny site (with diagrams) about how red cars can travel much faster than blue ones at http://www.bath.ac.uk/~ensrit/doppler.html - not exactly what your friend needs, but maybe good for a study break. Hope this helps - do encourage your friend to do a little investigating using AltaVista! -Doctor Sarah, The Math Forum Check out our web site! http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.