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Doppler Effect

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.


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 

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. 


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   :

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

There's a funny site (with diagrams) about how red cars can travel 
much faster than blue ones at 


 - 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/   
Associated Topics:
High School Physics/Chemistry

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