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How are human eyes able to detect the different curvatures of surface ? Basically how are the human eyes able to differentiate between a plane surface and a convex or a concave surface?

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    $\begingroup$ I'm voting to close this question as off-topic because it's more about Biology than physics. $\endgroup$ – Kyle Kanos Sep 28 '18 at 18:13
  • $\begingroup$ Curiously, the human eye feels comfortable with concave curvature, but not with convex. $\endgroup$ – FGSUZ Sep 28 '18 at 18:34
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This is possible because we have two eyes. They furnish depth perception, which lets us perceive curvatures.

Even with only one eye, we can be fooled into believing a perfectly flat 2-D surface is curved in 3 dimensions by processing cues of color, shading and perspective based on experience. This is the reason that we can "understand" a painting or a printed picture as representing 3-dimensional objects when the rendering itself is strictly 2-dimensional.

So, curvatures can be perceived because we have stereoscopic vision and because we have image-processing programs running in our brains that help identify curvature.

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Although stereoscopic vision can be useful for perceiving curvature of surfaces, in my experience shading (in the case of diffusive surfaces) is more important. Curvature in a specular surface is also easy to perceive using only one eye, because concave and convex mirrors impose different distortions on reflected scenes. Also, a person with vision in only one eye can easily learn to perceive curvature almost as well as a person with normal vision. Consider what you perceive when looking at a photograph of a curved surface. You perceive the curvature correctly most of the time, despite the fact that the photo was shot from a single point perspective. All that said, it's a bit easier to fool a single eye than a pair of eyes, into misinterpreting a scene. Bottom line: it is not so much a property of the eye or eyes that allows us to see curvature. Rather, it is our brain that makes it possible.

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