English: Source frame vacuum Doppler effect schematic for a rightward proper-speed of w/c=1[ly/ty] i.e. for a coordinate-speed of c/Sqrt[2], first at the instant of closest approach from the source-frame perspective (top center) to illustrate the transverse Doppler blue-shift, and then at the instant of closest approach from the receiver-frame perspective (top right) to illustrate the transverse Doppler red-shift.

The solid-red (source frame) and dotted-gray (receiver frame) arrow lengths and directions denote the relative wave-vector directions and spacial/temporal frequencies in these two frames, at these two instants. This figure is complementary to receiver-frame and source-frame all-angle plots also posted up here.

For the gray "receiver wave-vectors", source-frame closest approach occurs when the receiver sees the source as still oncoming, so that the detected photons appear more energetic (i.e. blue-shifted). For receiver-frame closest approach, however, the receiver sees the red-shift as arising not from longitudinal Doppler affects, but from time-dilation associated with the slower-running clock on the moving source. The converse is true for the red "source wave-vectors".