Question: What is the Doppler Effect?

Daniel Parsons answered on 11 Nov 2014:

Hello there,
The Doppler effect, named after the physicist Christian Doppler, is the change in frequency of a wave for an observer who is moving relative to its source. The best example is when an ambulance approaches, passes, and moves away from a static observer. Compared to the frequency of sound that the ambulance horn is sounding, the frequency heard by the observer is higher during the approach, identical when passing right by, and gets lower during as it moves away.

We use this a lot in the work that I do. We use an instrument called an ADCP – that is Acoustic Doppler Current Profiler – to measure how much water is flowing down big rivers. We use the Doppler effect to measure how fast the water is going by emitting and listening to the echo that come back. The change in frequency of the sound that is returned tells us how fast the water is going relative to us on the boat. We then use GPS to remove our boat motion from the measurements to get a good estimate of how fast the water is going and thus how much water in total is flowing down a river.

Last year in the Mekong River in SE Asia, during the monsoon, we measured a flux (or discharge) of 59,000 Cubic meters of water per second. That is a lot of water and about 200 times the size of the River Severn in the UK!