School of Physics Lecture Demonstration Manual

We-3 The Doppler Effect (with microwaves)

Published: Wednesday 15 March 2006 - Updated: Wednesday 30 March 2011


To demonstrate that a wave reflected from a moving object has its frequency shifted.



Click on pictures to enlarge.

Doppler Effect Diagram2


The Doppler alarm unit sends out a beam of microwaves. The beam reflected from a moving object (flicking a steel rule works well) is Doppler shifted. The original wave and its Doppler shifted wave are fed into a non linear device (a diode) which has as its output a component due to the difference frequency in the audible. This is amplified and clearly heard in a large lecture theatre.

Safety notes

Electrical safety 


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