1. Doppler Effect
The apparent shift in frequency caused by the movement of the sound source or the movement of the observer.
When the waves get crunched together (in front of the moving sound) the frequency of the wave is increased and the sound is high in pitch.
When the waves get spread apart (behind the moving sound) the frequency of the wave is decreased and the sound is low in pitch.

2. Doppler Effect
High pitch
Low pitch

3. Doppler Effect
Used in police radar to determine the speed of an oncoming car.
Used by meteorologists to determine the size and speed of an approaching frontal system.
Explains the sound you hear when listening to a Nascar race!