Doppler effect is the apparent change in frequency when there is a relative motion between the source and the observe.
As the source approaches
As the source moves away or recedes
V is the speed of sound
Vs is the sped of the source
Fs is the frequency of the source
F’ is the observed frequency by the stationary observer
The Doppler effect for electromagnetic waves such as light is of great use in astronomy and results in either a so-called redshift or blueshift. It has been used to measure the speed at which stars and galaxies are approaching or receding from us; that is, their radial velocities.
Red and blue shift
As the star moves away from the earth there will be increase in wavelength and decrease in frequency, this is redshift
As the star moves away towards the earth the wavelength will decrease and frequency will increase, this is blueshift
View the video on Doppler effect below