Relativistic Doppler effect

The relativistic Doppler effect is when sound waves or light waves appear to change in pitch or color because of how fast the thing that is making the waves is moving. For example, if a car is driving down the street, the sound of its horn will appear to get higher pitched as the car moves towards you, and lower in pitch as the car moves away. The same effect also happens with light waves, like if you're looking at a light that is moving towards you, the light will look more blue and if it is moving away it will look more red. This all happens because the fast movement of the thing that's making the waves changes how often the waves are reaching you.