Final answer:
When an emergency vehicle with a siren is approaching, you must pull over and yield the way to allow it to pass. The Doppler effect can help a driver recognize when the vehicle is approaching or has passed, as the siren's pitch changes. Regardless of these pitch changes, yielding to emergency vehicles is a legal requirement.
Step-by-step explanation:
If an emergency vehicle, such as an ambulance or fire engine, is sounding its siren and quickly approaching your vehicle from behind, you must pull over to the side of the road and stop if necessary to allow the emergency vehicle to pass.
This action is in compliance with traffic laws designed to clear the path for vehicles that are responding to emergencies. As the vehicle approaches, you may notice the Doppler effect, which is the change in frequency or pitch of the siren as the vehicle moves towards you and then away from you.
The siren will sound higher in pitch as it approaches and lower in pitch as it moves away. This is a physical phenomenon that occurs due to the relative motion between the source of the sound (the siren) and the observer (you, in your vehicle).
The Doppler effect aids in determining the direction of the approaching emergency vehicle. When you hear the siren's pitch rising, you can tell that the vehicle is approaching.
As the pitch begins to drop, you know the vehicle has passed by. Importantly, regardless of the pitch changes due to the Doppler effect, drivers are legally required to yield the way to emergency vehicles, acting quickly and safely to facilitate their passage.
For pedestrians or observers at a fixed location, the point at which the frequency they hear most closely matches the actual frequency of the siren is when the emergency vehicle is directly in front of them – this is when the vehicle's velocity relative to the observer is zero, minimizing Doppler shift.