What does the Doppler effect describe?

1. The effect appears whenever the source of a wave and the observer move relative to each other. 2. What the observer measures is a different frequency from the one the source is emitting. 3. So the Doppler effect is a change in the observed frequency of the wave. 4. The speed of sound in the medium is not altered by the motion of the source. _Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Chapter 6: Doppler effect_