# Doppler Effect

This effect is described as a characteristic observed in waves emitted or reflected by moving sources relative to the observer.

The effect was first described theoretically in 1842 by Johann Christian Andreas Doppler, named after the Doppler Effect in his honor.

For sound waves, the Doppler effect is the phenomenon whereby an observer perceives different frequencies than those emitted by a source and occurs due to the relative velocity between the sound wave and the relative movement between the observer and / or the source.

Whereas:

We can determine a general formula for calculating the frequency perceived by the observer, ie the apparent frequency.

• Assuming the observer is at rest and the source moves:

For the case where the source approaches the observer, there is a shortening of the wavelength, related to the relative velocity, and the real frequency will be lower than the observed one, ie:

But as the source moves, its speed must also be considered, so that:

Replacing in calculating the observed frequency:

That is:

For the case where the source deviates from the observer, there is an apparent wavelength elongation, in which case the deduction of the observed frequency calculation will be analogous to the previous case.

However:

So:

 We can write a general formula for cases where the source shifts and the observer stands still if we use:Being the negative sign used in the case where the source approaches and positive in the case where the source moves away.