It is known an object put at the focal length of a convex lens isn't visible since there aren't any light rays that intercept either real or extended. And it is known the eye is a convex lens. So what would happen if an object is put at the focal length of the eye? Would we be able to see it? Would its width matter, even if not visible if it is so wide it could cover all the light rays coming into our eyes, reminding that there is some thing in front of us?

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    $\begingroup$ Also note - there is not one focal length for the eye. The lens of the eye is flexible, and the focal length of that lens varies depending on how close an observed object is to the eye. $\endgroup$ – David White Apr 26 at 17:01

When rays diverge from a point at the focal length of a lens, they end up parallel at the far side of the lens. In order to see the point source, your eye must be able to cause the rays to converge to a point on the retina, which is on the back surface of the eye. But if the rays are not parallel they do not converge, so your eye does not form an image on the retina. Of course light will reach the retina and will be perceived, but there will not be a point image.


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