Q & A: The difference between a mirror and a photograph

Q:
I am near sighted. When I look into a mirror, everything close is in detailed focus, but distant objects look fuzzy (just like if I am looking directly at them, rather than at their refections). My question is: since mirrors do not reflect as near or far sighted, why do distant objects still look fuzzy to me even thoogh I am standing very close to the mirror.
- Bill Moody (age 69)
gulf shores
A:

Great question—you can learn a lot about optics by thinking this through.

First, let's talk about why you're nearsighted. (I am, too.)

Let's say we have a pointlike light source like a small bulb. It emits light in all directions. To make an image of the bulb with a camera or your eye, you need to focus that light back to a small point. Any convex lens can do this. However, only a lens with a particular strength will focus the light from a bulb at a particular distance in front of the lens to a small point at a particular distance behind the lens. In a camera, this distance needs to be equal to the distance between the lens and the film for an image to appear "in focus." In your eye, it's the distance between the lens and your retina (the light-sensitive layer on the back surface of the eye).

In a camera, you can usually change the distance between the lens and the film a little bit to change the distance to objects that will appear in focus. The lens in your eye is able to change shape and become stronger or weaker to focus light from different distances onto the retina. If you're nearsighted, the distance between your lens and your retina is a little too long, and your lens can't adjust itself enough to focus far-away objects at the correct distance. The light from far-away objects focuses a little bit in front of the retina instead, then spreads out and appears blurry.

Now let's consider the mirror. Light obeys a simple rule when it hits a flat mirror: it reflects with the same angle at which it hit the mirror.

After the light has reflected off the mirror and followed this rule, you can see that it happens to be arranged exactly like light that came from a source behind the mirror. In fact, your eye can't tell the difference. It focuses the reflected light exactly like it would focus light that came from a distance equal to the distance between the bulb and the mirror + the distance between your eye and the mirror. If you're nearsighted, and this distance is farther than you can normally see clearly, the bulb will look blurry.

The rule of reflection is what makes a mirror different from a photograph. A photograph scatters light in all directions, like our bulb. So the light from a photograph of a bulb looks just like the light from a real bulb, and your eye will focus it as if it came from the distance between the photograph and your eye. A mirror reflects light that comes in at a particular angle in just one direction, so it can preserve information about distance.

Rebecca H.

PS: Here's something else to puzzle about—why are reflections in mirrors reversed left-right but not up-down?

(published on 02/16/2016)