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enter image description here

Here the object is AB kept at first focal point F₁, and we see the light rays emerging parallel to each other, meeting at infinity.

When the object is kept at focus of a convex lens we say that image is formed at infinity. According to classical optics if image is to be formed then at least two rays must intersect. But in this situation we don't see any two rays intersecting. Nevertheless we can see the image.

So this might mean that that parallel rays are intersecting at infinity but where exactly?

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  • $\begingroup$ where do you think they intersect, if anywhere? $\endgroup$ Oct 10, 2021 at 11:28
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    $\begingroup$ You don't see the image unless you add another lens. This may be the eye lens. $\endgroup$
    – nasu
    Oct 10, 2021 at 11:45
  • $\begingroup$ You can't see an image in that case. The lens will just be filled by a more or less uniform mush of colour. $\endgroup$ Oct 10, 2021 at 11:45
  • $\begingroup$ They don't Meet at INFINITY. They meet at any finite distance when we see image. $\endgroup$
    – Anomalous
    Oct 13, 2021 at 6:57
  • $\begingroup$ One way to look at "focus at infinity" is to think of a lens with the object at the focus as a system that converts spatial position to angle. Consider the tip of the arrow in your diagram. As I change the length of the arrow, rays starting at the tip (regardless of what direction they leave the tip) always form parallel rays after the lens, but the angle of the parallel rays changes. $\endgroup$
    – garyp
    Oct 13, 2021 at 10:59

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Firstly, you must remember that the formula which predicts that the rays will meet at infinity assumes an ideal lens and environment- in real life the lens will not be perfect, so the rays will not emerge exactly parallel, and the environment in which the lens exists will absorb and scatter the light rays.

That said, saying the rays will meet at infinity means that they will not, for all practical purposes, meet unless another lens is placed in their path. If that lens is your eye, for example then the rays will be focussed to form an image on your retina.

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