Re: Does the eye focus differently on mirrors than on pictures?

The question is: "I am nearsighted. If I look at an image on the surface of a mirror, the objects in the 'background' are blurred but the objects in the 'foreground' are in focus. Why? It is a two-dimensional image. When I look at a two-dimensional picture, all of the objects are in focus. Why is this not true when I focus on an image on the surface of a mirror?"

The short answer is that the image you see of the world in a flat mirror is NOT two-dimensional!!

There are actually several useful answers already on our site:

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, but I will make a stab at my own explanation, too, and you can take the information from all the answers!

The main reason for what you have noticed is that the image of the world, as seen in a flat mirror, is not actually ON the surface of the mirror. The dust particles, scratches, smudges, etc., that you see ON the surface are what your eye focuses on if you intend to see these things. But if your attention shifts to the scene that is being reflected to you, your eyes will be focused closer to "infinity", or far distance. The "image" that you see "in" a flat mirror is "virtual" (an optical term), which means that, although you can "see" an image with your eyes (or a camera), the image itself, without additional optics, can not be seen on a white screen; thus the image is not "real" (another optical term). The optics of your eyes are necessary for an image to form on the retinas, which is a real image. A camera would do the same thing with the rays from the mirror, except the real image would be formed onto the film (or the detector, if it's a digital camera).

Our eyes take the reflected light rays from the mirror and make images on our retinas. The light rays are identical to, but switched left and right from, the rays that your eyes would catch if you were to turn around from the mirror and look directly at the objects that you were previously viewing "in" the mirror.

The "image" you "see" in a flat mirror appears to be the same distance behind the mirror as the objects that make up the image are in front of the mirror. That's why your eye can not focus on the scene and the mirror's surface at the same time. I was discussing this with my very perceptive 15-year- old son, and he points out that it is exactly the same as looking through a glass window: you can either focus on the glass itself or you can focus on the scene beyond the glass, but you can't simultaneously focus on both (unless you are so far away from the window glass that it is "far" and is roughly in focus with the scene beyond).

The two-dimensional picture is actually ON the flat surface on which the picture is made, whether it's canvas or a photograph, etc. It is a flat two-dimensional representation of what our eyes would see, but it is a fake! By that I mean that you are not actually looking at the real objects, and there is nothing behind the picture from which any light rays can come which would allow your eyes to form an image from the distance that the picture is pretending to show.

Happy imaging!! - - John Link, MadSci Physicist