Mirrors are used by eye doctors for their standard eye tests nowadays, if their office doesn’t have the luxury of 20-foot long rooms.
They’ll have a mirror on a wall in front of you, and project a mirror image they want you to look at on the wall behind you. The room’s arranged so that the distance from the image on the wall behind you to the mirror then back to your eyes is 20 feet.
As Cecil said, the fact that the light rays had to reflect off a mirror doesn’t affect how much your eyes need to focus, which is the cumulative 20 feet.