|I wear glasses for distance vision, but my near vision is good. Why is it that when I use a nearby mirror to view distant objects, I must wear my glasses to see them clearly? I should be able to see the nearby mirror well without glasses. — JFJ
When you view something in a flat mirror, you are looking at a virtual image of the object and this virtual image isn't located on the surface of the mirror. Instead, it's located on the far side of the mirror at a distance exactly equal to the distance from the mirror to the actual object. In effect, you are looking through a window into a "looking glass world" and seeing a distant object on the other side of that window. The reflected light reaching your eyes has all the optical characteristics of having come the full distance from that virtual image, through the mirror, to your eyes. The total distance between what you are seeing and your eyes is the sum of the distance from your eyes to the mirror plus the distance from the mirror to the object. That's why you must use your distance glasses to see most reflected objects clearly. Even when you observe your own face, you are seeing it as though it were located twice as far from you as the distance from your face to the mirror.