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I am a high school student and I have a question related to the concept of object and image I know where incident rays intersects this point is called object but here if this is the case

problem

here what will the observer see? for sure light says are entering into his eyes and are also converging at a single point on his retina and sending signals to the brain but what output the brain will give? how will it decide from where these are originating from?

Qmechanic
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Arun Bhardwaj
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  • Do I understand you right? You want to know what the ey sees without the second lens? then why did you include it in your sketch? If you want an answer pease indicate the focal point or distance in your sketch, I guess it ist about 3/4 of the distance from A to the lens? – trula Aug 20 '21 at 10:26
  • Ya I shouldn't have include the second lens, and it is just a rough diagram I just want to know if our eyes are placed in between the point of intersection{c} and lens facing the lens what would we see? – Arun Bhardwaj Aug 20 '21 at 11:19

4 Answers4

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how will it (the brain) decide from where these are originating from?

The brain doesn’t have to “decide” anything. Light rays from different points on the object will be brought to a focus at different points of the image (the diagram shows the paths of just two light rays which originate from the same point). In normal vision the lens of the eye can make this image lie on the retina. The muscles of the eye can make small changes to the shape of the lens to bring the image into focus on the retina (and specifically around the fovea, which is the most important part of the retina for detailed vision). This adjustment of the lens is called accommodation

With short sight or long sight the image may be brought into focus in front of or behind the retina, and the eye needs some help to bring the image into focus on the retina. This is what external lenses in spectacles do. External lenses may also be used to make images larger than they would normally be so that the brain can see more detail in the image - this is how magnifying glasses, binoculars, microscopes and telescopes work.

But in any case, all the brain perceives is an image on the retina. The brain does not need to “decide” how far away the object is (although there are ways it can do this if you are using both eyes, or if you move your viewpoint) and does not know anything about any lenses between the eye and the object.

Obviously the brain can tell something about the shape and direction of the source object from its image (although optical illusions exploit the ambiguities in this process). But if the brain could tell the distance and size of the source object from a single image then pictures, computer screens and televisions could not fool the brain into perceiving a three dimensional scene instead of a patchwork of colours on a flat surface. And moving your viewpoint around only helps you determine the relative distance of two objects, not their absolute distance. To determine absolute distance you would need to use some sort of active imaging system such as sonar.

gandalf61
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  • what? do u mean the brain doesn't tell us the position, shape and size of the source? I don't know how it tells us the position of the object but for sure we can tell the distance of the object from us no matter if I use one eye or both because if I close my one eye and just look from one the position shifts to left or right but still the distance remains the same – Arun Bhardwaj Aug 20 '21 at 12:53
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    @ArunBhardwaj I have added a paragraph to my answer to explain further. An image on the retina gives the brain no information about the absolute size and distance of the source object. That is why it is so easy to trick the brain into perceiving a three dimensional scene that isn't there at all. – gandalf61 Aug 20 '21 at 14:07
  • but do you know how its decide the distance of the source? lets make it simple if I am seeing a pen ,I think our brain tells us the shape and size of it by making some output out of where those light lays coming from different points of pen hits on different-different points on the retina? that's why we see size of any object getting increase as it moves toward my eyes but actually its not, I don't know how it gives an idea of the distance ,it cannot be from retina because I have myopia and I can see distance of the pen same as any normal person would see – Arun Bhardwaj Aug 21 '21 at 14:08
  • and can u answer this question because this is even more confusing https://physics.stackexchange.com/questions/658655/why-does-an-image-only-form-where-light-rays-coming-from-a-single-point-get-refl/659323?noredirect=1#comment1481456_659323 – Arun Bhardwaj Aug 21 '21 at 14:10
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The observer at O will see a highly defocused image, and their eye lens will not be able to accomodate to get the image into focus. Let's assume the viewer has normal vision, and is not severely nearsighted or farsighted. For that viewer, objects at infinity are focused at the retina, and form an image image there. Now consider near objects: Objects closer (say at the end of your arm) would focus beyond the retina, but the eye lens (if working properly) accomodates (which causes an increase in power) and the image moves from beyond the retina to the retina. So the near objects (as long as they are not too close) are in focus.
But the way you have drawn the setup, the light from the first lens is converging. This light will focus in front of the retina. The eye lens will not be able to accomodate in the negative direction. The image in front of the retina will appear out of focus on the retina.
All of this can be changed if someone is severely nearsighted or farsighted, but that's a different problem. If someone were farsighted, it is possible that converging light enters their eye (as you have shown) and that would form a sharp image. Of course, the details matter of how farsighted they are, etc...

JB2
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I remember having the same question at school. The problem is, all the diagrams given in textbooks have objects at infinity/on the axis. In reality, the light coming from the top and bottom of the object make slightly different angles with the lens and are focussed at different points

Toby Peterken
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to know the distance of an object who picture is on the retina you need two eyes. depending of the distance of the eyes there is a difference between the two pictures and the brain "calculates the distance. Try to meet the points of your finger looking with only one ey, and the will only meet if you adjust more and more, so you have to do it fast, than try the same with to eyes open. If you ever have seen a 3d picture or film with special glasses they project two different pictures to your eyes. With one ey you estimate distance since you know their actual size. in the actual picture you postet, the ey can not see the point A, since the real picture is at point C, so your ey is not able to see A as a point. take any convex lens you own, first decide its focal length, by projecting a picture of a far away thing, best through your window. the sharp picture will be at about the focal distance. Next look at some letters in about 1.3 to 1.5 of the focal length, and your ey closer than 1.5f, you can not see the letters sharp.

trula
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