The Doppler Effect
The Doppler effect is a beautiful principle of relativity and wave frequency. It is the change in wavelength of a wave for an observer moving relative to the source of the wave. The received waves on a siren, for example, on a passing emergency vehicle of sorts will start out at a higher frequency as the vehicle approaches the observer, will be equal to the transmitted waves at the moment of passing the observer, and will lower in frequency during the recession. It is not actually the frequency that changes, itis the wavelength. So, the siren on an ambulance will start out higher than its stationary pitch while approaching you, will be at its stationary pitch while next to you, and will be lower than its stationary pitch as it passes away from you.
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