The pitch of the sound emitted by the siren of a moving fire engine appears to change as it passes a stationary observer - Leaving Cert Physics - Question c - 2012

The Doppler effect can be explained through the following points:

1. Moving Wave Source: When the fire engine (the sound source) moves towards the stationary observer, it compresses the sound waves in front of it, leading to a higher frequency or pitch that the observer hears.

2. Wavefronts: As the fire engine approaches, the distance between the wavefronts decreases, resulting in an increase in pitch. Conversely, as the fire engine moves away, the wavefronts spread out, causing the pitch to decrease.

Diagram

<--Wave Approaching -->
      |     /
      |    /  --  Direction of Motion
      |   / 
Source -->

3. Observer's Perception: The observer hears a higher pitch as the sound waves are closer together when the source is approaching, and a lower pitch when it is moving away.

No, the crew in the fire engine will not notice this phenomenon because there is no relative motion between them and the sound they are emitting. They are moving with the sound source, thus they do not perceive a change in pitch.

The Doppler effect is widely applied in various fields:

• In radar systems, to measure speed by detecting frequency changes in the reflected waves.
• In medical imaging, using Doppler ultrasound to study blood flow.
• In astronomy, to measure the speed and distance of stars through redshift and blueshift of light.