Doppler effect

The Doppler effect is a change in frequency and wavelength of a wave. It is caused by the change in distance between the thing creating the wave (causer) and whatever is measuring seeing or hearing the wave (watcher or observer).^[1]

Another word for "causer" is "sender" or "source". Another word for "change in distance" is "speed" or "relative velocity". A common example is sitting in a car while another car goes by the watcher. The watcher will hear a change in pitch, while the sender will not.

For waves read all the waves that can send out or reflected by an object. And for reflection read change of direction of a wave.

1. ↑ Wragg, David W. (1973). A Dictionary of Aviation (first ed.). Osprey. p. 112. ISBN 9780850451634.