1. Doppler Effect The change in the frequency of a wave received by an observer (detector) compared to the frequency with which it was emitted is known as the Doppler Effect (in honor of the Austrian physicist, Christian Doppler, who did pioneering work on the topic). The Doppler Effect (also called Doppler shift) takes place whenever there is relative motion between emitter (or source) and receiver (or observer, or detector).

2. The wavefronts are called condensations in the diagram. As long as there is no relative motion between source and observers, the two observers on either side receive waves at the same frequency and wavelength. Picture is take from Cutnell and Johnson - Physics 8e. Doppler Effect – No relative motion

3. Doppler Effect– Stationary Source Picture Source: Physics, 4e by James L Walker.

4. Doppler Effect– Stationary Source

5. Picture Source: Physics, 4e by James L Walker.

6. Doppler Effect– Stationary Source

7. Doppler Effect– Moving Source The observer at the right measures a shorter wavelength – hence a higher frequency as the source approaches; while the observer at the left measures a longer wavelength – hence a smaller frequency as the source moves away. Picture from Cutnell and Johnson

8. Doppler Effect– Moving Source Picture Source: Physics, 4e by James L Walker.

9. Doppler Effect– Moving Source

10. Doppler Effect Combining all four cases