![]() Conceptual Explanations The Doppler effect is a given change in the frequency of a wave in a medium due to the velocity of either the source, observer, or both. Common applications of this effect in everyday life include hearing a car pass you on the interstate, or as you are waiting for the crosswalk to allow you to cross. More applications include medical imaging (specifically in echocardiograms), as well as police, ambulance, and military sirens. The effect itself manifests in two ways: changes in frequency based on emissions that travel through a medium, such as air, and through waves that can propagate in a vacuum. One way to think about this difference is simply remembering that sound waves propagate through air, and light/electromagnetic waves propagate in a vacuum. In air, the Doppler Effect is going to be the perceived difference in frequency via the combination of waves as they are emitted from the source. As the source is moving with some velocity v towards you, the observer, the sound wave that is emitted is going to be released closer to you than the previous waves, so the new and old waves are going to combine, thus increasing the frequency of these waves. This is why, as the source gets closer to you, the noise sounds like it has a higher pitch. However, once the source and the observer are at the same location, this effect will instantaneously dissipate since there are no waves that are combining to make an increased perceived frequency. Once the source starts moving away from observer in the opposite direction, the perceived frequency actually decreases. This is so because the source is no longer “pushing” the waves into each other to make a higher frequency, but instead, these waves are spread out, reducing the time it takes for the observer to hear them, causing a decrease in observed frequency. For our purposes, we are only going to discuss light as an example of the Doppler effect in a vacuum (we could discuss gravity, but that involves the theory of general relativity, which is beyond the scope of this project). Since space is a vacuum, the most applicable usage of the Doppler effect in a vacuum is indeed in space. Two phenomena, redshifts and blueshifts explain this as a means of measuring planetary velocity. A redshift simply means that as a light source moves away from an observer, the wavelength is going to decrease, just like sound waves decrease in frequency as they move away from an observer. As the source of light moves away from the observer and the wavelength increases, the emitted light becomes more red as noted by the spectrum of visible light ROYGBIV. However, this is not necessarily an absolute rule, as invisible light undergoes this process as well, it just simply means that as the wavelength increases, so does the ‘redness’ of the emitted light. A blueshift, on the other hand, means the exact opposite. In this case, as the source of light moves towards the observer, the light perceptively becomes ‘more blue’ and increases with wavelength. The Doppler Effect also has many more applications of usage, as will be discussed further in this webpage. Observed Frequency (f) is related to Emitted Frequency (f0) by: f = f0 (c+vr) (c+vs) Where: c = wave velocity in the medium vr= velocity of the object, relative to the medium
Source of Image (above): http://www.grc.nasa.gov/WWW/k-12/airplane/doppler.html |
Derivation Doppler Effect Formula (Shown Below)
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Source used in Document Above:
Halliday, David, Robert Resnick, and Jearl Walker. Fundamentals of Physics. 7th ed. Vol. 1. Hoboken: John Wiley & Sons, 2005. Print. |