Correct Answer: The Doppler effect is the change in frequency (and wavelength) of a wave in relation to an observer moving relative to the wave source.
Step 1: Understand that waves can be sound waves or light waves.
Step 2: Know that the Doppler effect happens when the source of the wave is moving.
Step 3: Realize that if the source moves towards an observer, the waves get compressed, making the frequency higher.
Step 4: If the source moves away from the observer, the waves get stretched, making the frequency lower.
Step 5: Remember that this change in frequency also affects the wavelength of the waves.
