When an ambulance approaches the sound of its siren becomes higher pitched, i.e. its wavelength decreases, and when it moves away it becomes lower pitched, i.e. its wavelength increases. A similar effect occurs with light. When a star moves away the wavelength of its light increases. Check the difference in the sound of the siren when the ambulance is approaching and when it is moving away.
Introduction to Deep Earth Science
Sensing Planet Earth – Water and Ice
Introduction to Astrophysics