Answer: The Doppler effect occurs when a source of a wave is moving relative to an observer (or the observer is moving relative to the source). In this case, the apparent frequency of the sound, as heard/seen by the observer, is shifted with respect to the original frequency of the wave.
More specifically, the relationship between the apparent frequency, f', and the original frequency, f, is given by:
where:
is the velocity of the wave
is the velocity of the observer relative to the source, and it is positive if the observer is moving towards the source, and negative if the observer is moving away from the source
is the velocity of the source relative to the observer, and it is positive if the source is moving away from the observer, and negative if the source is moving towards the observer
The doppler effect occurs in many daily-life situations: for instance, when an ambulance approaches you, you hear an increase in the apparent frequency of the siren due to the Doppler effect. Another example is the movements of distant galaxies from us: when they move away from us, the apparent frequency of the light they emit decreases, so their wavelengths appear to increase towards the red color (red-shift); on the contrary, when they are moving towards us, the apparent frequency seems to increase, so the wavelength seems to decrease towards the blue color (blue-shift).
Explanation:
