The Doppler Effect is something which occurs when something which emits sound or light moves relative to an observer. The object, observer, or both can move, causing an apparent change in the frequency of the wavelengths being emitted by the object. The Doppler Effect explains why a rude driver's car horn appears to change in frequency as he or she zooms by while leaning on it, and an understanding of this effect can help scientists make a variety of observations about the world around them.
This effect was proposed and described by Christian Doppler in 1842. As often happens, the first one on the scene got to name the effect, so the phenomenon is now known as the “Doppler Effect.” Many people are familiar with the basic concept, even if they don't quite understand how it works. The short version of the story is that when an object approaches an observer, the wavelengths are compressed, causing the frequency to increase, and as the object moves away, the wavelengths spread out, causing a corresponding decrease in frequency.
When an object is approaching an observer, the frequency of the wavelengths appears to increase, in a phenomenon known as blueshifting. As the object moves away from the observer, the frequency seems to decrease from the observer's perspective, in a situation known as redshifting. Another example commonly used to illustrate the Doppler Effect is the siren of an emergency vehicle. As the vehicle approaches, the pitch of the siren seems to change, causing it to sound higher, and when it moves away, the siren starts to sound lower.
This interesting effect is not just intriguing to observe. Knowledge about the Doppler Effect can allow people to use it for the purpose of taking measurements. Being aware of the shift in frequency caused by this effect, for example, can allow astronomers to determine how far away stars are, and whether they are growing closer or further away relative to Earth. The ability to map the stars with the Doppler Effect has allowed astronomers to propose a number of theories about the universe, including the idea that the universe is slowly expanding.
In medicine, the Doppler Effect is used in medical imaging to gather data during real time imaging studies. Because the frequency of wavelengths can be precisely measured and described with the right equipment, people can use changes in that frequency to understand how something works, if they know the proper equations to use to understand the implications of those changes. Numerous other fields of endeavor take advantage of the Doppler Effect to gather or distribute information.