According to the doppler effect what appears to happen when a light source moves closer to an observer

When the distance between the source and the observer is DEcreasing, the observer sees light that has a shorter wavelength than the light actually had when it left the source.

It doesn't matter how the distance between them is decreasing.  The source could be moving toward the observer, the observer could be moving toward the source, or they could both be moving in a way that makes the separation between them decrease as time goes on.  The result is the same.

(When this is used in Astronomy and Astrophysics, the big trick is:  How does the observer know what the wavelength actually WAS when the light left the source ?)

explanation:

newton's third law, for every action there is an equal and opposite reaction.