The Doppler Effect

When the source moves away from the observer the frequency as measured by the source will be smaller. This is known as the Doppler effect.

Astronomers call the increase in wavelength due to Doppler effect as red shift since a wavelength in the middle of the visible region of the spectrum moves towards the red end of the spectrum.

When waves are received from a source moving towards the observer, there is an apparent decrease in wavelength, this is referred to as blue shift.

The formula given above is valid only when the speed of the source is small compared to that of light.