You have probably heard of the Doppler Effect in conjunction with recent improvements in radar technology: "Doppler Radar" is capable of measuring the velocities of winds, and is instrumental in the identification of tornados. The basic principle is familiar to you when driving as well: the pitch (frequency) of the horn or siren of an approaching vehicle is higher than when it passes you and recedes. This "Doppler Shift" in the frequency also has an important medical usage in the measurement of the speeds of moving fluids inside the body.
When an object which emits a wave (sound or light), the frequency is determined by the object itself. The wavelength, however, is a function of the speed of propagation in the medium as well as the motion of the object within the medium. When the source of the wave approaches you at a speed u s, the wavelength is shortened by an amount u s T, where T is the period of the wave. This is simply due to the motion of the source: when the wave cycle started, the source was at point "a", but when the cycle ends the source has moved u s T closer. (See a 37K quicktime animation). Since the "received" wavelength is related to the "source" wavelength by
= l s - u s l s / c
= l s (c - u s) / c,
The same principle applies when the source is stationary but you are approaching it at a speed u r. Now the received wavelength is related to the source wavelength by
= l s c / (c + u r)
The next section is about the physics of hearing.
If you have stumbled on this page, and the equations look funny (or you just want to know where you are!), see the College Physics for Students of Biology and Chemistry home page.
©1996, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.
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