Why does a siren's pitch change as it passes by?

The pitch of a siren changes as it passes by due to the Doppler Effect.

The Doppler Effect is a phenomenon in physics that describes how the frequency of a wave changes for an observer moving relative to the source of the wave. In the case of a siren, the sound waves it produces are compressed as it approaches you, and stretched as it moves away. This change in frequency is perceived as a change in pitch.

When the siren is approaching, the waves are compressed because the source of the sound is moving towards the observer. This means the waves reach the observer more frequently, resulting in a higher pitch. Conversely, when the siren is moving away, the waves are stretched out, meaning they reach the observer less frequently, resulting in a lower pitch. This is why the siren seems to have a higher pitch as it approaches, and a lower pitch as it moves away.

This effect is not limited to sound waves, but applies to all types of waves, including light and radio waves. For example, astronomers use the Doppler Effect to determine whether stars and galaxies are moving towards or away from us, by analysing the change in frequency of the light they emit.

The Doppler Effect is named after Christian Doppler, an Austrian physicist who proposed this idea in the 19th century. It's a fundamental concept in wave physics, and is crucial in many areas of science and technology, including radar, sonar, medical imaging, and even GPS systems.

In summary, the change in pitch of a passing siren is a practical demonstration of the Doppler Effect. The relative motion between the source of the sound (the siren) and the observer (you) causes the frequency of the sound waves to change, which we perceive as a change in pitch.