| DOPPLER EFFECT, the change in the
observed frequency of a wave produced by motion of the wave’s source receiver,
or both, first discussed by Christian Johann Doppler (1803 - 1853) in 1842.
The effect is particularly noticeable with sound waves, as, for example,
a change in the apparent pitch of the bell or whistle on a passing railroad
train. The origin of the effects seen in which a source of waves is shown
moving with a velocity v towards the right, away from 3 stationary receiver.
At the end of an interval t, the source has moved a distance vt; and if
1 is the length of the waves that it produces, the number of waves strung
out in space between source and receiver has increased by vt/l. If the
waves are emitted at a frequency fe, then fet waves are emitted in a time
t. The number of waves received, however, frt, is smaller than that emitted
by vt/l. This leads to the relation
fr = fe - v/l which also holds true when the receiver moves while the source is at rest. When v is much smaller than the speed of sound c, 1 may he approximated by its uncorrected value c/fe with negligible error. The received frequency is lower than the emitted frequency when the source and the receiver are moving apart, and higher when they are approaching each other. Motion of the medium that is transmitting the sound waves (such as wind blowing toward or away from the receiver)also causes a change in the received frequency. The Doppler effect is of importance in astronomy sonar, and radiolocation. Its usefulness in astronomy stems from the fact that the velocity of a star along the line of sight can be determined from the Doppler shift in the spectral frequencies of the light which it emits. The most striking instance of this is in the light from very distant galaxies here the so-called "Red-Shift" indicates that they are all receding from us at speeds that increase with their distance up to nearly half the speed of light. Whether the universe is actually exploding in this way, or whether the red-shift is due to some cause other than the galaxies' motion, is still an open question. Radiolocation is the location of objects, usually aircraft or missiles, by bouncing radio waves of high frequency from them and detecting the radio waves as they return. If the target is moving either towards or away from the apparatus at high speed, the received signal will exhibit an appreciable Doppler shift on its return, and the amount of this shift can be used to estimate the speed of the target. The same Doppler shift is observed with high-frequency sounds and is used in the underwater location of a moving submarine. |