Final answer:
When encountering emergency vehicles with active sirens and lights, pull over and stop to let them pass. The Doppler effect explains the pitch change of the siren: it's highest when approaching, lowest when departing, and matches the actual siren frequency when the vehicle is directly in front of the observer.
Step-by-step explanation:
When you encounter emergency vehicles such as police cars, fire trucks, ambulances, or volunteer fire/rescue squad members with their sirens and lights on, the correct action to take is to safely pull over to the nearest edge of the road and stop, allowing these vehicles to pass swiftly. Not only is this the law in many jurisdictions, but it can also mean the difference between life and death for those in need of emergency assistance.
In terms of acoustic physics, this scenario involves the Doppler effect, which describes the shift in frequency or pitch of a sound wave relative to the observer who is moving relative to the sound source. When an ambulance is coming towards you, its siren will sound higher in pitch due to the Doppler shift, and as it goes away from you, the pitch will decrease. The frequency you observe will most closely match the actual frequency of the siren when the ambulance is directly in front of you because at that moment there is no relative motion between you and the ambulance along the line of sound propagation.
The Doppler effect can be observed in everyday life with passing emergency vehicles, as well as with other moving sound sources such as cars, airplanes, and trains. It is characterized by a noticeable change in the frequency of the sound as the source moves closer and then further away from the observer.