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We know that the lens in our eyes forms an inverted image on retina and brain reverses it to give the correct image. But when one wears specs the image would get reversed twice so we should see things inverted. Specs forms inverted image on lens -- lens again inverts it on retina -- brain again inverts it. Please explain.

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  • $\begingroup$ Your assumption about the behavior of "specs" is unjustified. If you can prove your assertion about the image being reversed or inverted by them, show that. $\endgroup$ – Bill N Dec 3 '18 at 20:50
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When you have two lenses that are close to each other, to an approximation they function as a single lens whose diopter value is the sum of the individual lenses' diopters. Here, "close" means "at a distance small relative to the lenses' focal lengths". Only when lenses are "far apart" will they introduce new behavior (such as image inversion).

Typical (convex) eyeglass lenses have diopter values of 3 or less, which corresponds to a focal length of 1/3 meter or more, which is far longer than the distance between the glasses' lens and the lens in your eye. So, the lens doesn't add an inversion.

You can try this yourself with a (convex) eyeglass lens. Hold it close to your eye and you'll see the world slightly magnified, with the focus closer than usual. Now move the lens slowly away from your eye, and you'll see the image magnify further and further, until there will be a point where it's all a blur. Moving it further still, and you'll see the image come back, inverted. The distance where you only see a blur for distant objects is the focal length of the lens.

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