I am a high school student and I am a little confused about a thing I know generally when we say point object we meant that the size of the object is very small compared to the distance we are looking from but in geometrical optics if the meaning of point object remains the same then there would be many points in front of the object where light rays would meet and when these rays will enter into our eye then shouldn't we perceive that point instead of seeing the finite sized point?or are these points not enough intense means not enough light lays will enter from these points into our eyes? so that we can see them and we perceive? I am confused in these limits that a point means what?

point object

and if my reasons were correct then will we perceive these points if we have a highly intense source of light say a bulb in "vacuum" {because in presence of air around the bulb would scatter light and we would see air}? I think we should perceive these points in vacuum too just like we perceive any virtual/real image.

and I think if anyone who can answer this question can answer my this question too Why does an image only form where light rays coming from a single point get reflected or refracted and converge to a common point? and let me know if the reason that I mentioned which is that not enough light rays from those points are entering in our eye then how there it is valid? because there should be more rays intersecting at those points if we have light rays coming from more points from the object {say a ROD}.


(a) Good idea to ask only one question at a time.

(b) Remember that light rays are just directions of travel of waves.

(c) When we look at any scene, even using only eye, waves scattered from different points enter the eye and some of their directions of travel will intersect, since the width of the pupil is non-zero. But the optical system of the eye focuses light from one point on the scene to a point on the retina. It is unaffected in this behaviour by the crossing of the paths of light rays from different points on the scene. The crossing-points are not imaged on the retina; for one thing the eye is presumably set to bring to focus on the retina light from points on the scene before it, which will be further away than the crossing points. Your small objects that are quasi-points are just small parts of the scene and are included as such in the ramblings above.

Comments on your question: "Why does an image only form where light rays coming from a single point get reflected or refracted and converge to a common point?"

The diagrams show rays that come off at angles from different points on an extended object such that they pass through the same point outside the object. Note that there will be no fixed phase relationship between waves reaching this point but originating at different points on the extended object.

On the other hand, waves originating from a single point and brought to a focus by a lens will arrive at the focus (image point) in phase, as one can understand by looking at a wavefront diagram of a converging lens forming a real image of a point object. So there will be reinforcement of these waves, in a way which doesn't happen for waves from different points on an extended object that just happen to cross at a point.

  • $\begingroup$ "The crossing-points are not imaged on the retina" yes that's what I want to know why it doesn't affect ? the only reason I can think of is its not that intense i.e not many light rays are coming from it, but I don't know how or eye lens accomodates for random moving light rays and focuses on the retina, it surely cannot get focused at just one point that point of focus too will be large $\endgroup$ Aug 30 '21 at 7:27
  • $\begingroup$ "its not that intense i.e not many light rays are coming from it" I think that's important. Light will be scattered in many directions from a point on an object, and nested cones of rays will enter the pupil. Two rays from different points on the object that happen to cross before entering the pupil will not form an image on the retina, even if the eye were adjusted to accommodate the point of intersection – which is unlikely. $\endgroup$ Aug 30 '21 at 17:55
  • $\begingroup$ but if that's true then what would happen if there are sufficient light rays from that point? then we should see those points instead of seeing the object directly because finally light rays are entering into our eyes from those points but that doesn't happen why? $\endgroup$ Aug 31 '21 at 6:13
  • $\begingroup$ "what would happen if there are sufficient light rays from that point?" There won't be. $\endgroup$ Aug 31 '21 at 18:25
  • $\begingroup$ but at least think , how do our eyes can treat the light rays coming from each point separately and focuses them separately over the retina? why it doesn't focuses the light rays coming from the those points on the retina? if it would focus then we will definitely not be able to see the object itself because then whole light rays from all points will get combinedly focused on the retina and I hope u will understand what I am trying to say $\endgroup$ Sep 3 '21 at 9:43

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