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Think of a time you've heard a race car, ambulance or train pass. When the vehicle is coming towards you it sounds high pitched. Once it passes where you are the pitch sounds lower. This is known as the Doppler Effect.
Sound waves in front of the object are compressed, making their wavelength shorter (and the pitch higher OR light is more towards the blue end of the spectrum).
Sound waves behind the object are stretched apart and have longer wavelengths (and lower pitch OR light is more towards the red end of the spectrum).
Stationary Sound Source
Sound waves travel in all directions with the same wavelength
Source Moving Slower than the Speed of Sound
.
Source Moving at the Speed of Sound
Mach 1 - Breaking the Sound Barrier
Source Moving Faster than the Speed of Sound
> Mach 1 - Super Sonic.
This is what causes the shockwave known as a sonic boom.
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