Concept:The Doppler Effect states that relative motion alters the perceived frequency of a wave.
Solution:- As the police car (source) moves rapidly towards the listener, it "chases" the sound waves it just emitted.
- This forward motion physically squashes the wave fronts together in the space ahead of the car, shortening the spatial wavelength (\( \lambda \)).
- Because wave speed in air remains constant (\( v = f\lambda \)), a shorter apparent wavelength forces the listener's ear to register a higher number of wave impacts per second.
- Therefore, the perceived frequency (pitch) strictly increases.
Why other options are incorrect:Frequency would only decrease if the car were driving away (receding). It stays the same only if the car is parked.
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