Have you noticed how the pitch of a police car or ambulance siren changes as it passes where you are standing, or how an approaching car or train sounds different to when it is traveling away from you? If you haven’t, try to do an experiment by paying extra careful attention the next time it happens to see if you can notice a difference in pitch. This doesn’t apply to just vehicles and trains but anything that emits waves, be those sound waves or any other electromagnetic (EM) waves.
The effect actually occurs if you move towards or away from the source of the sound as well. This effect is known as the Doppler effect and will be studied in this section.
Optional Investigation: Creating the Doppler effect
You can create the Doppler effect with some friends. One way of doing this is to get:
- string, and
- a tuning fork.
Tie the string to the base of the tuning fork. Strike the tuning fork to create a note and then hold the other end of the string and swing the tuning fork in circles in the air in a horizontal plane.
The string needs to be very securely tied to the tuning fork to ensure that it does not come loose during the demonstration.
The class should be able to hear that the frequency heard when the tuning fork is moving is different to the frequency heard when it is stationary.
The Doppler effect is named after Johann Christian Andreas Doppler (29 November 1803 – 17 March 1853), an Austrian mathematician and physicist who first explained the phenomenon in 1842