The Doppler effect was discovered by Christian Doppler in 1842, and describes how waves seem to change with motion. With sound waves, the Doppler effect describes the familiar shift in pitch as a car passes by you (ask any five-year-old the noise a car makes, and you’ll hear the NEEEEOUUUUWWWWW of the Doppler Effect.)
With light waves, the Doppler effect describes the redshift and blueshift of stars as they move towards or away from us in space. That particular Doppler effect makes the characteristic absorption lines of different chemicals appear in different places, which we can measure. Unlike many things in astronomy, the Doppler shift effect doesn’t get stronger or weaker with distance. This means we can use the same trick to determine the minute shifts of a star’s velocity due to a planet orbiting it, AND the rate of expansion of the universe based on the motions of galaxies that are billions of light years away.
This powerpoint presentation demonstrates the Doppler effect. While it may look like the circles representing sound and light waves aren’t actually just expanding circles, they really are – the strange way they look is just the Doppler effect in action. It might even be worthwhile to show people what the slide looks like with all its animations, before running it.