I have a quick relativity question. Say an observer is moving at a relativistic velocity with respect to some light source that emits light at one particular frequency (the observer's path does not intersect the emitter). At the point of closest approach, will light from the emitter be Doppler shifted in the observer's reference frame? To illustrate what I'm talking about, I've attached an image, where the observer's velocity is parallel to the x-axis, and at closest approach, its separation from the emitter is parallel to the y-axis. Anyway, I'm tempted to say that there's no Doppler shift, since the emitter's velocity with respect to the observer is parallel to the observer's velocity. But I'm told that this isn't correct. Does anyone know what's going on here?