What is the X in X-mas?
Doppler effect n.
An apparent change in the frequency of waves, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart.
|Doppler effect |
The difference between the frequency of a wave (as of sound or light) as measured at its source and as measured by an observer in relative motion. The Doppler effect can be used to determine the relative speed of an object by bouncing a wave (usually a radar wave) off the object and measuring the shift in the frequency of the wave. This technique is the basis of Doppler radar, as used in traffic control and navigation systems. The Doppler effect is also known as the Doppler shift. ◇ If the source and the observer are getting farther apart, the observed frequency is lower than the source frequency. In the case of light waves, the phenomenon is known as red shift. The amount of red shift in the spectra of stars is used in astronomy to determine how quickly the Earth and those stars are moving apart. ◇ If the source and the observer are getting closer together, the observed frequency is higher than the source frequency. In the case of light waves, the phenomenon is known as blue shift.
Our Living Language : The whistle of an approaching train has a higher pitch as the train approaches than when it recedes, even though that same whistle, heard by a passenger on the train, maintains a constant pitch. This is an example of the Doppler effect, common to all wave phenomena (in this case, a sound wave). Motion toward the source of a wave (or, equivalently, motion of the source toward the observer) entails that the peaks and troughs of the wave are encountered more quickly than if there were no motion, so the frequency of the wave is higher for the moving observer (hence the higher whistle pitch). Similarly, motion away from the source entails following the wave's motion, so the peaks and troughs are encountered less often, and the frequency is lower for the moving observer (hence the lower whistle pitch). The Doppler effect on light waves has enabled scientists to determine that the universe is expanding. The frequencies of light given off by various substances (such as the burning of hydrogen in the fusion reactions of most stars) has been found to be lower in distant galaxies and other celestial objects, a phenomenon called red shift, since the visible light is shifted toward the red, low-frequency end of the spectrum. Astronomer Edwin Hubble reasoned that the red shift was due to the Doppler effect. As galaxies speed away from us, the frequency of the light emitted appears lower. Doppler radar and sonar use the Doppler effect on reflected radio and sound waves to distinguish between stationary and moving objects and to determine the velocity of moving ones; the echolocation of bats and some whales also exploits the Doppler effect on reflected sound waves for navigating and catching prey.
A phenomenon observed with waves. The frequency of a wave of light or sound seems higher if the source is moving toward the observer and seems lower if the source is moving away. For example, if an automobile blows its horn as it travels past someone, the apparent pitch of the sound will be higher as it approaches the person and then will grow lower as it passes and moves away.