Why is it that when I’m looking at a mirror with my glasses off, everything is blurry, even though the mirror is really close to me?
In: Physics
The lenses of the glasses are designed to help correct for light incoming at various angles, focusing them in tandem with the lenses in your eyes to form an image on your retina.
The issue is that the mirror isn’t the origin of the light you are seeing, instead light that reflects from the mirror retains the angle it came in at. This is why by changing your viewpoint will change what you can see in the mirror. But this also means that your glasses are needed to correct for the angle of light coming from distant objects reflected in the mirror as if they were actually the combined distance away of your distance to the mirror plus the distance of the mirror to the object.
Mirrors from a physics perspectives don’t work like an object on the wall in front of you. In light-terms, the light is coming off that object 6 feet behind you, bouncing off the mirror, and then back into your eye.
Effectively, the mirror is a “copy” of the light behind it, and works as if the things 6 feet behind you are “6 feet behind” the mirror itself. Thus the maximum focus length you can get without your glasses is the same (actually probably less) looking in a mirror as looking behind yourself.
If you were 100 miles away from the mirror and turned on a torch / held up a sign, would you expect someone standing by the mirror to be able to see it/read something? No. Same thing.
The mirror is just duplicating what you’d see if you turned round.
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