The physics
At a plane mirror, the angle of incidence equals the angle of reflection. We measure both from the normal. The normal is the line at 90° to the mirror.
i = r one mirror turns a ray by δ = 180° − 2i
Turn a mirror by α and the reflected ray turns by 2α. Two mirrors at an angle θ turn a ray by 360° − 2θ.
δ = 360° − 2θ
A plane mirror's image sits behind it, as far as the object is in front. Slide the mirror, and the image moves twice as far.
Two mirrors at an angle θ make many images. With n = 360°/θ, an even n gives n − 1 images. An odd n gives n − 1 on the bisector and n off it.
Under water, a thing seen from above looks d/μ deep. A mirror images the thing at that apparent place.