Editing Relativistic Doppler effect (section)

==== Point of null frequency shift ====
[[File:Transverse Doppler effect scenarios 6.svg|thumb|300px|Figure 4. Null frequency shift occurs for a pulse that travels the shortest distance from source to receiver.]]
Given that, in the case where the inertially moving source and receiver are geometrically at their nearest approach to each other, the receiver observes a blueshift, whereas in the case where the receiver ''sees'' the source as being at its closest point, the receiver observes a redshift, there obviously must exist a point where blueshift changes to a redshift. In Fig.&nbsp;2, the signal travels perpendicularly to the receiver path and is blueshifted. In Fig.&nbsp;3, the signal travels perpendicularly to the source path and is redshifted.

As seen in Fig.&nbsp;4, null frequency shift occurs for a pulse that travels the shortest distance from source to receiver. When viewed in the frame where source and receiver have the same speed, this pulse is emitted perpendicularly to the source's path and is received perpendicularly to the receiver's path. The pulse is emitted slightly before the point of closest approach, and it is received slightly after.<ref name="Brown_b">{{cite web |last1=Brown |first1=Kevin S. |title=The Doppler Effect |url=https://www.mathpages.com/home/kmath587/kmath587.htm |publisher=Mathpages |access-date=12 October 2018}}</ref>