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## Physics - Doppler Effect

Suppose a police car loudly sounding its siren is approaching a passenger standing in the street. When the police car is approaching, the passenger hears its siren having a high pitch. But, as it passes by the siren’s pitch falls off. That is because of the Doppler Effect.

The Doppler Effect arises whenever a source of waves is moving in relation to its observer or an observer is moving in relation to the source of waves. The result is that the waves will have different wavelengths and frequencies than expected. When a source of waves approaches an observer, the waves crunch up meaning that from the observer’s view point, the waves have shorter wavelength and a higher frequency. As a source of waves recedes from an observer, the waves stretch. This means from the observers viewpoint the waves have longer wavelength and lower frequency.

Rule number 41: Doppler Effect

The Doppler Effect is summarized in terms of frequency in the following equation. The frequency observed by the observer equals the actual frequency of sound being emitted times the velocity of sound in air plus or minus the velocity at which the observer is moving over the velocity of sound in air plus or minus the velocity at which the source is moving. Notice that if neither the observer nor the source is moving then the frequency observed is equal to the actual frequency.

You will have to predict when to choose plus or minus. When the observer approaches the source, the frequency observed must increase. When the source approaches the observer, the frequency observed must increase. When the observer recedes from the source, the frequency observed must decrease. When the source recedes from the observer, the frequency observed must decrease.