How is this possible re bizarre optical behavior of mirror I bought

I’m 45 years and my vision is now not only nearsighted (glasses since age 13), but it’s getting harder to see things close up as well. I normally need my glasses for anything past about 1 foot from my face at this point, and closer than 7 inches starts getting blurry as well. To help see things close up for personal grooming, I purchased this neat little magnifying mirror the other day that’s slightly concave and gives approx 2X or so magnification.
After playing with the mirror for the first time I set it on a table about 2.5 feet away and had not yet put my glasses back on, I then I looked at the mirror and the image it reflected of the living room about 20 feet away was perfectly clear without glasses! It’s a bizarre experience (for me) to be seeing clearly at a distance without glasses. When I put my glasses on the image blurred. It’s freaky!

What’s going on?

A magnifying mirror has an optical effect the same way a magnifying lens does - that is to say, a mirror with curvature has an optical effect. This one happens to deliver about the effect that your eyes require to work well.

As I understand the optics of vision, you could have a different prescription for your glasses that would work at a distance of six inches rather than the one you currently use that is tailored for sitting on your nose. The farther away the glasses were the more the curvature of the lens would have to change.

A curved mirror acts similarly to a lens, you just see stuff on your side of it rather than on the other. You seem to have found the distance at which the mirror matches the prescription on your glasses. My guess is that if you move closer or farther from the mirror it will not be as clear, and that you probably have a similar prescription for each eye (or the mirror would only be clear in one).

I appreciate the reply, but I guess I should of have been more clear. I’m really looking for fairly specific technical explanation of how this is happening when the mirror is 2.5 feet away from face, given that my glasses normally need to be an inch or so from my eye to work.

Thanks! I just noticed the mirror image is flipping everything upside down in “clear” mode as well. Interesting!

Just a quick explanation, as I have to run – your glassses correct your vision by adding optical “power” (the lens focuses light). The mirror that you bought magnifies because it, too, has optical power. Evidently the effect of looking at things through the mirror is to bring things to a point at which your natural power of accommodation (fine-tuning of the fovus using your eye’s own lens) allows you to see things clearly.


I don’t think that’s correct. The OP is nearsighted, and the lens used for nearsighted people is shaped like this )( — the opposite of a magnifying glass, which has a shape like this (). It therefore actually defocuses light.

The eyeball of a nearsighted person is one in which the focus attained by the eye’s lens is in front of the retina, i.e., the lens is too “strong” or the eyeball too long. The lens prescribed for a nearsighted person compensates for this, moving the focus backwards to where the retina is so as to attain focus.

You’re quite right that some lenses – including those to correct myopia – are negative. But I think that the OP is mistaken about the mirror he’s using. The mirror’s I’ve seen that allow you to see your whole body are convex, not concave. That is – they look like the outside of a spoon, not the inside. If the mirror was indeed concave, your image would be upside-down. For a full-body mirror, you want a rightside-up image. So you use a convex lens (You can’t use a flat mirror if you want to see your whole body – that will only let you see half of your body at a time, regardless of how far away or how close you hold it. This is a classic optics problem). If this is true, then the mirror has negative power, just like the lenses needed to correct myopia, so the myopic person using one can see distant items more clearly.

You’re probably correct, but in glancing at the mirror I could have sworn the mirror looked like it curved in a bit, not out. I see your point though re the spoon scenario. I’ll confirm when I’m back at home tonight. Thanks for reply!

Would the fact that the object in question is a magnifying mirror and not a magnifying glass have anything to do with it? Since most Mag Mirrors i’ve seen are infact concave. )(

I am no expert in optics, but since he claims the view is upsidedown, doesn’t this mean the focal point of the mirror is somewhere right in front of him? In this sense, there should be a point just before the focal point and just after that compensates for his prescription, and would appear crystal clear. The point before the focal point “>” would be upside down, the point “<” after (in between the focal point and the mirror) would be right side up. Both of these points would be of equal distance from the focal point “.” of the mirror “)”.

> . < )

Please correct me if i missunderstand.

As a general rule, inverted images are real images. For a mirror, this means that the image is in front of the surface of the mirror, not behind it as it would be for a plane mirror (contrary to popular belief, the image is never on the plane of the mirror, unless the object is, too). So the image can be close enough for you see comfortably, even if the mirror isn’t. Incidentally, we can also conclude from this behaviour that the OP’s mirror is concave, and for objects inside the focal length of the mirror, the image will be magnified.

My point was not to compare or dispute the OP’s description of the mirror as concave but rather to question CalMeacham’s assertion (as I understood it) that the clearer vision obtained was due to the mirror, like the lens of the OP’s glasses, “increasing the power” of the light by focusing it.

Whether due to a lens shaped like this )( or to the surface of a mirror shaped like this --> ( {shiny side being pointed at}, the refraction that helps a nearsighted person see in focus does so not by concentrating / focusing the light towards a point but by doing the opposite, so as to compensate for our natural lens focusing light too soon (in front of the retina).

Excuse my cluelessness re this topic. I know everyone is trying to be helpful, but I’m not quite sure I grok the why of it yet.

I have a concave magnifying mirror approximately level with the suface of my eyeball sitting 2.5 feet away +/- on a desk. On the suface of that mirror is resolved a clear upside down image ofa scene 20 feet away. I am shocked by this in that normally I can’t see anything clearly without my glasses, and yet there it is, a razor sharp upside down image in the mirror I can see without my glasses.

I guess the most surprisng this is that I’m used to things more than a foot away being fuzzy, and it’s a bit of a shock to see this clear somewhat distant image in the mirror.

I assume the light ray path is that the reflected light from the object strikes the mirror’s surace and then stikes my eye. What is the mirror doing to that normally fuzzy image (to me) that makes it appear sharp as a tack to my nearsighted eye 2.5 feet away.

The key point is that the image you’re seeing is not somewhat distant. The image is close to you; closer to you than the mirror is. Exactly how much closer we can’t say, without knowing more about the mirror and the distance of the object (the living room you’re seeing). But it could in principle be within a foot of your face (or whatever distance you can comfortably see).

The mirror that astro describes is indeed convex - only a convex mirror will give a magnified image when you are up close to it. The image in a concave mirror always looks smaller (i.e. wing mirrors on cars with the stickers warning you that what you see is closer than it appears)

A convex mirror has the property of being able to focus the rays of light reflecting from it to a point on the same side of the mirror as the object. This it the principle that reflecting telescopes use. Although the surface of a cosmetic mirror such as astro describes is almost certainly not ground to exacting tolerances, when you place your eyes at the focal point of the mirror you can see a reasonably sharp image of distant objects. Exactly where a myopic person (like astro, and me too) needs to place their eyes will be slightly different than where a far sighted person, or one with normal vision, but it will work the same for everyone.

If you want to get more detailed information, try searching for newtonian reflectors or parabolic mirrors, I’m sure there are many amateur astronomers out there with webpages devoted to the topic.

I told you I was in a hurry (had to get to a meeting) - I was using “focus” in the broad sense of changing the collimation or not of a bundle of light rays, so it could defocus as well, as you note.

I still haven’t had a chance to look the OP over properly. Suffice it to say that the power of a lens or mirror is not sufficient to determine whast it does. Given two positive lenses I can, by varying the spacing between them, make a combined lens with greater positive power, equal positive power, negative power, or no power at all. You gotta look at the whole setup, and spacing is critical.

moes lotion, you have “convex” and “concave” reversed, in your otherwise correct answer. Just remember: A cave is concave.

When you look at an object, your eyes are seeing the light rays that started at the object and diverged (in straight lines in all directions) from that object. What mirrors and lenses do, in general, is to bend those light rays, so that they appear to come from somewhere else. The “image” formed by a mirror or lens acts like a new object, possibly of a different size and orientation, and in a different place than the original object. So when you look in a flat mirror, you see images which act, optically, exactly as if you’d placed a mirror-reversed object the same distance behind a hole in the wall. Note (this is the important part) that this is different from a painting on the wall. If you get within a foot of a flat mirror on the wall, you’ll be able to see the mirror’s frame clearly, but the faraway objects reflected in the mirror will still be out of focus, because their images are still far away.

For flat (and convex) mirrors, the images all happen to be behind the mirror (and so, farther away from your eyes than the mirror itself). For concave mirrors, this is not always the case. A concave mirror can focus light from faraway objects at a point between you and the mirror, forming an image that is closer to you than the mirror is. In this case you can have a situation where the mirror frame is out of focus (too far away) but the images of faraway objects, somewhere between you and the mirror, are in focus.

Notice that the distance to the mirror is not directly important; what’s important is the distance to the image. (Given a particular mirror, these two distances are related; I’m just saying that you will be able to see focused images as long as the images are sufficiently close to your eyes, regardless of the distance to the mirror.) In the same way, how close your glasses are to your eyes is not directly important in determining whether objects seen through them will be in focus. (If your eyes were trying to focus on your glasses, how would contact lenses work?)

I have a similar experience when looking at the convex mirror I stuck on the driver’s side mirror of my car. See my earlier post on this with concise answers.