When I take photographs of a subject reflected by a surface (water, mirror, glass) I notice that the focussing distance is not the distance from my camera to the surface, but mostly it is close to infinity.
For example: From a distance of 1 meter I take a photo of a reflection in a small water puddle. Autofocus sets the focus to infinity and not to ~1 meter.
I recall from basic physics that mirror images are "virtual" images of the real object, but I am not sure anymore how that works. I would be glad if someone could explain why I have to focus on infinity and not on the reflective surface itself.