If the observer and the source are receding from each other. The Doppler effect is a phenomenon observed with all waves. Whenever a source generating a wave moves relative to an observer or an observer moves relative to a source, the frequency of the wave at the location of the observer is shifted relative to the frequency of the source. The frequency at the location of the observer increases when the source and observer are approaching each other, and decreases when they are moving away from each other. For waves traveling in a medium the frequency of a wave at the location of an observer is given by the two formulas above, with v being the speed of the wave in the medium, v 0 the speed of the observer relative to the medium, and v s the speed of the source relative to the medium.
Electromagnetic waves can travel through empty space.
For EM waves traveling through empty space, the frequency of a wave at the location of an observer is given by , if the observer and the source approach each other with relative speed v. If the observer and the source recede from each then their relative velocity v in the above formula is a negative number.