[Halc]

It doesn't apply to horizontal or vertical mirrors,

Why do you think that is?

Angle of incidence is the same as angle of reflection of a vertical or horizontal (relative to the motion) mirror in a light clock.  The angle doesn't change due to relativistic contraction, but it does for any other clock angle.

My statement is not necessarily true if the light source is not proper-perpendicular to the mirror, since any mirror can be made vertical with an arbitrary designation of which way is up.
Proper-perpendicular means the incidence angle is zero in the frame of the mirror, which it is in any light clock.

[Fussball (OP)]

Does relativistic beaming play a part in reflections? It seems to me that it does. Relativistic beaming is relativistic reflection, right?

[Fussball (OP)]

Angle of incidence is the same as angle of reflection of a vertical or horizontal (relative to the motion) mirror in a light clock.  The angle doesn't change due to relativistic contraction, but it does for any other clock angle.

Yes, but why doesn't relativistic law of reflection apply to horizontal mirrors? Why shouldn't it apply to horizontal mirrors?

[Halc]

Does relativistic beaming play a part in reflections?

I'm unfamiliar with the term.

Angle of incidence is the same as angle of reflection of a vertical or horizontal (relative to the motion) mirror in a light clock.  The angle doesn't change due to relativistic contraction, but it does for any other clock angle.

Yes, but why doesn't relativistic law of reflection apply to horizontal mirrors? Why shouldn't it apply to horizontal mirrors?

Well, it does, but it just yields the same number for incidence and reflection.

[Fussball (OP)]

I'm unfamiliar with the term.

https://en.wikipedia.org/wiki/Relativistic_aberration

"In this circumstance, the rays of light from the source which reach the observer are tilted towards the direction of the source's motion (relative to the observer). It is as if light emitted by a moving object is concentrated conically, towards its direction of motion; an effect called relativistic beaming. Also, light received by a moving object (e.g. the view from a very fast spacecraft) also appears concentrated towards its direction of motion."

[Fussball (OP)]

Well, it does, but it just yields the same number for incidence and reflection.

Why does it yield the same number?

[Halc]

Well, it does, but it just yields the same number for incidence and reflection.

Why does it yield the same number?

In a frame where a mirror is moving parallel to its surface, the mirror acts identically to a stationary mirror.  You can shave using the reflection of a shiny train as it passes you at any speed.  Thus incident and relection angles are the same (regardless of incident angle) because that's what happens with the stationary mirror.

If a frame where a mirror is moving perpendicular to its surface, light coming in parallel to the motion will hit the mirror perpendicular (incident angle of zero) and thus reflect the light straight back the way it came (reflection angle of zero).  If, however, the light does not come from that way, the incident and reflection angles will not be the same for a mirror moving in any manner not parallel to its surface.