Increase or decrease in wavelength as the object emitting the wave moves relative to the observer. For example, a vehicle turbine whine seems to be higher in pitch when the vehicle is approaching the observer (because the waves are compressed, shortening the wavelength), and lower in pitch when it is travelling away (the waves are elongated, lengthening the wavelength). With light waves when the light source is coming or going relative to the observer the same thing happens. For example, when a ship is moving away from a star, the star's light appears redder, as the light waves are red shifted, meaning the wavelength is longer. When a ship is moving towards a star, the light appears bluer, as the light waves are blue shifted or compressed and shortened.