The Doppler Effect
This is what you see:
The blue dot is the source of sound. The green line is you, listening to the sound.
This is what you do:
Click PLAY and the sound waves are seen moving out as red circles. Note that when the source velocity is 0 and the observer velocity is 0, the circles are concentric, so the wavelength to the right of the source is the same as the wavelength to the left of the source.
Now set the source velocity to + .4 and run the simulation. What do you observe about the relative wavelengths to the right of the source compared with the wavelength to the left of the source?
Also notice what happens to the wavelength YOU OBSERVE while the source is approaching you compared to what you after the source has passed you and is moving away.