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If you like riddles, you'll like this one: How can a musician playing a single note on a horn change that note without changing the way he or she plays that note? At first, you might think this is a trick question. Clearly, the musician must do something to change the pitch, right? Wrong. If the musician plays the same note while moving toward or away from a stationary listener, the note heard by the listener will indeed change -- even if the musician does nothing different. Dutch scientist Christoph Hendrik Diederik Buys Ballot conducted this very experiment in 1845. He assembled a group of horn players and placed them in an open cart attached to a locomotive. Then he had the engineer start up the locomotive so it could carry the cart, complete with the horn players, back and forth along the track. As they were being pulled, the musicians played a single note on their horns.
Ballot stationed himself beside the track and listened carefully, both as the train approached and receded. And the notes he heard were different than the notes being played by the musicians. The phenomenon is called the Doppler effect after Austrian mathematician Christian Johann Doppler, who first predicted this odd behavior of sound in 1842. Today, scientists know that the Doppler effect applies to all types of waves, including water, sound and light. They also have a good idea why the Doppler effect occurs. And they've incorporated its principles into a variety of useful tools and gadgets. In this article, we'll examine everything Doppler: the man, the science and the technologies. But first we have to lay some groundwork. Because the Doppler effect is a phenomenon associated with waves, let's start by covering some basics about the two basic types of waves -- light and sound. But light and sound also travel as waves. A light wave, like a water wave, is an example of a transverse wave, which causes a disturbance in a medium perpendicular to the direction of the advancing wave.
In the diagram below, you can also see how transverse waves form crests and troughs. The distance between any two crests (or any two troughs) is the wavelength, while the height of a crest (or the depth of a trough) is the amplitude. Frequency refers to the number of crests or troughs that pass a fixed point per second. The frequency of a light wave determines its color, with higher frequencies producing colors on the blue and violet end of the spectrum and lower frequencies producing colors on the red end of the spectrum. They are longitudinal waves, created by some type of mechanical vibration that produces a series of compressions and rarefactions in a medium. Take a woodwind instrument, such as a clarinet. When you blow into a clarinet, a thin reed begins to vibrate. The vibrating reed first pushes against air molecules (the medium), then pulls away. This results in an area where all of the air molecules are pressed together and, right beside it, an area where air molecules are spread far apart.
As these compressions and rarefactions propagate from one point to another, they form a longitudinal wave, with the disturbance in the medium moving in the same direction as the wave itself. If you study the diagram of the wave above, you'll see that longitudinal waves have the same basic characteristics as transverse waves. They have wavelength (the distance between two compressions), amplitude (the amount the medium is compressed) and frequency (the number of compressions that pass a fixed point per second). The amplitude of a sound wave determines its intensity, or loudness. The frequency of a sound wave determines its pitch, with higher frequencies producing higher notes. For example, the open sixth string of a guitar vibrates at a frequency of 82.407 hertz (cycles per second) and produces a lower pitch. The open first string vibrates at a frequency of 329.63 hertz and produces a higher pitch. As we'll see in the next section, the Doppler effect is directly related to the frequency of a wave, whether it's made of water, light or sound.
This source produces a series of wave fronts, with each moving outward in a sphere centered on the source. The distance between wave crests -- the wavelength -- will remain the same all the way around the sphere. An observer in front of the wave source will see the waves equally spaced as they approach. So will an observer located behind the wave source. Now let's consider a situation where the source is not stationary, but is moving to the right as it produces waves. Because the source is moving, it begins to catch up to the wave crests on one side while it moves away from the crests on the opposite side. An observer located in front of the source will see the crests all bunched up. An observer located behind the source will see the waves all stretched out. Remember, the frequency equals the number of waves that pass a specific point per second, so the observer in front actually sees a higher frequency than the observer in back of the source.
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