Doppler effect

The apparent change in frequency of sounds or light waves that accurs when the source of the waves and the observer are moving relative to one another. It is named after Christian Johann Doppler, who first described it in 1842. The effect is most commonly experienced with sound, as for example when an ambulance or police car passes by with siren blaring. The frequency, or pitch of the sound when the vehicle is approaching is noticeably higher than when it is receding.

The sound waves approaching are effectively bunched closer together, or shortened (higher frequency) , while those receding are effectively drawn out, or lengthened (lower frequency). The same effect occurs with starlight. The light from an approaching star has an apparently shorter wavelength, or looks bluer than it should. The light from a receding star has an apparently longer wavelength, or looks redder.

Source Image : Wikipedia