The best scenario to describe the doppler effect would be listening to the siren of a passing ambulance or fire truck
then it is coming towards you, the pitch is higher, it gets higher as it approaches and peaks as it gets right in front of you. then it drop at once when it passes you and continues to drop till it fades away. this is a classic descrption of the doppler effect
' A ' is one crest of the wave. After every wavelength, there's another one.
' B ' . . . the vertical arrow under B shows the amplitude of the wave
' C ' is one trough of the wave. After every wavelength, there's another one.
' D ' . . . the horizontal arrow over D shows the wavelength.
