The Doppler effect (1)

Nee-nah, nee-nah! The sirens wail as the police car/ambulance/fire engine whizzes past you on the street. But the pitch of the siren changes: it's higher as the siren approaches you, and lower when it's heading away from you again.

That, my friends, is the Doppler effect in action. Sound is composed of waves of compressed air. Because the siren is moving towards you at speed, the sound waves are compressed slightly more when they hit your ear drum. And, similarly, when the siren is moving away, the sound waves are slightly decompressed by the movement away.

A little bit of physics to keep you on your toes!