Based on what you have seen in this Gizmo, what is the best definition of the Doppler effect?

Change in the frequency and wavelength of wave is called Doppler effect. The distance between the the origin of wave and the person hearing or seeing the wave is the main cause of Doppler effect. Example. When a car goes by a person who is sitting in a car (watcher) then the he will notice a change in the pitch of the car while the person in the car will not hear any change. The sound at the origin have high pitch while its pitch becomes lower at a distance from the origin.