The Doppler Effect

The doppler effect  is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession

This is a short clip of sheldon from The big bang theroy giving an example of what the doppler effect is