![]() Remember that the frequency of light in the visible spectrum determines its color. Redshift and Blueshift in the Doppler Effect in Light So for low relative speeds, the Doppler effect in light is very similar to the Doppler effect in sound. This is actually the exact result for the Doppler effect for sound waves (but then where \(c\) is the speed of sound). This formula relates the frequency that the observer observes, \(f_\). ![]() We can now formulate the formula of the Doppler effect in light. Thus, if we call the speed of light \(c\) (as everyone does) and the speed between the observer and the emitter \(v\), then we can define the speed between the observer and the emitter as a fraction of the speed of light as The Doppler effect in light all has to do with the speed of light, particularly the speed between the emitter and the observer relative to the speed of light. The Formula of the Doppler Effect in Light In this sense, the Doppler effect in light is simpler than the Doppler effect in sound. The speed of the objects with respect to the medium in the case of sound is essential, while there is no medium at all for light waves. This specific Doppler effect is possible with sound but not with light, Wikimedia Commons CC BY-SA 3.0.įrom the example above, we see that there are not only quantitative differences between the Doppler effect in light and sound but also qualitative differences in some circumstances. This specific effect can't even happen in the case of light because emitters and observers can never go faster than the speed of light: there will always be light waves in front of the emitter and light waves will always be able to reach the observer. This effect is demonstrated in the animation below where the emitter clearly outruns the sound waves so there are no sound waves in front of it. In turn, this means that the sound will never even reach the observer who is in front of the emitter. The emitter can emit a sound, but the speed of sound is smaller than the speed of the observer! This means that there are no sound waves ahead of the emitter: they all trail behind the emitter, even though the sound waves are propagating in the same direction as the emitter is traveling in. However, if the emitter and the observer are both jet planes, where the emitter is flying behind the observer at the same supersonic speed, we have a whole different situation. Then in the case of light, there is no Doppler effect. Suppose the speed between the emitter and the observer is zero.
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