Final answer:
The wavelength of the police car’s siren sound decreases as it approaches due to the Doppler effect, causing a higher pitch, and increases as it moves away, leading to a lower pitch. This effect also applies to light in terms of blueshifts and redshifts in astronomy.
Step-by-step explanation:
When a police car with its siren blaring approaches you, the wavelength of the sound waves decreases, and as it moves away, the wavelength increases. This is due to the Doppler effect, which is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. As the police car comes closer, the sound waves are compressed, resulting in a higher frequency and a higher pitch of the siren's sound. When the car moves away, the sound waves are stretched, leading to a lower frequency and a lower pitch. This change in pitch is akin to a blueshift when the police car approaches (waves become more frequent) and a redshift when it moves away (waves become less frequent).
Additionally, if we were to observe light instead of sound, the same principle would apply. Light would exhibit a blueshift if the source moves towards us and a redshift if it moves away, although this is typically more relevant to astronomical observations than to police cars.