RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA / SCIENCE PHOTO LIBRARY RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA / SCIENCE PHOTO LIBRARY
Police car siren demonstrating the Doppler effect. As the police car moves, the sound waves from the siren become spread out behind it and compressed in front of it. This creates a change in pitch as the vehicle moves past an observer. It will sound higher in pitch as it is approaching the observer and lower in pitch as it is receding from the observer.
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