To see an object, its light rays have to meet on the retina in the focal point. But the focal point is a small white dot. Basically nothing would be distinguishable and the retina would burn because all energy is concentrated at a single point? Obviously, this is not happening. But why?
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3$\begingroup$ Because the people whose retinas got burned out all died. $\endgroup$– WillOMar 26 at 19:07
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3$\begingroup$ > "To see an object, its light rays have to meet on the retina in the focal point." This seems wrong. Instead, a seen object has a small image on the retina. Only point source maps to a point image; extended source maps to extended image. Source? $\endgroup$– Ján LalinskýMar 27 at 1:47
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6 Answers
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The following diagrams show how the eye focuses:
There may be people with vision problems such that the focal point is a single point on the retina, but this can be corrected with glasses.
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5$\begingroup$ The trouble with the diagrams, especially the first one, is that it suggests that light from the top of the tree only passes through the top of the lens, comes together at a focus in the middle of the eyeball and then spread out again to form an image. That is deceptive and the last sentence is incorrect. No lens can ever bring light from different parts of an object together at a single point. $\endgroup$– James KMar 27 at 5:41
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$\begingroup$ There may be people with vision problems such that the focal point is a single point on the retina, but this can be corrected with glasses. Is this condition harmful for the retina when not wearing glasses? $\endgroup$– StefanHMar 27 at 13:17
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In ordinary life, only a point object will focus to a point image. Otherwise the light is spread out, just as the object is spread out. But a very intense object can injure your retina: don't stare at the sun!
With lasers, collimated light simulates a point object at large distance. Lasers can be seriously hazardous to your vision.
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Even with ideal vision, all the light from an extended object is not focused onto a single point in the retina. Instead each point on the object is brought to a focus at a different point on the retina - the image of the whole object is spread out. So only very bright objects (such as the sun) can cause damage to the retina.
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$\begingroup$ Doesn't this imply that the focal point must be slightly in front of the retina and not directly on it? $\endgroup$– StefanHMar 26 at 17:44
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$\begingroup$ @StefanH No. A simple lens dos not have a single focal point. Its focal point depends on the direction of the incoming rays. These focal points all lie on a focal plane. With ideal vision this focal plane falls on the retina. $\endgroup$ Mar 27 at 7:37
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$\begingroup$ Then the pictures given by @Xavier confuse me on that. In these pictures, the focal point (where the rays meet) seems to be in front of the retina... $\endgroup$– StefanHMar 27 at 13:28
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$\begingroup$ @StefanH You are misreading the diagrams. A focal point is a point where all the different rays from one point on the object converge. These points all lie on the retina - this is clearer in the second diagram, where rays 1 and 2 from the top of the tree converge at a point on the retina. $\endgroup$ Mar 27 at 20:41
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$\begingroup$ I see. I think I confused what is often called the focal point as the point where all rays entering in parallel (coming from infinity) are concentrated. But if I got it right, every point on one side of the lense has it unique focal point on the other side. So, there are infinitely many focal points for every lense, and the the focal point is just picked out because rays coming from infinitey are somewhat special. $\endgroup$– StefanHMar 28 at 13:04
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Part direct answer, part reply to your follow-up question on Gandalf's answer, "Doesn't [light from an extended object not focusing on a single point] imply that the focal point must be slightly in front of the retina and not directly on it?"
Concentration of rays on a single point in geometric optics is an approximation. A target and a light source are in thermal contact. If we could make energy go from the source to the target such that the target was hotter than the radiation temperature of the place where the light was originally emitted, we would be transporting heat from a cold region to a hot region without doing extra Work, which violates the 2nd Law. In optics this limitation is expressed as the conservation of optical etendue. The light from a given source can't be more squished together than the the light emanating from the surface of that source. This is why very bright (the sun) and very squished (a laser) light sources are both hazardous.
(Note as regards the temperature of sources and targets that the light from a laser is emitted from deep inside the tube where the extremely hot lasing medium is, not the comparatively cool tip where the light comes out of the tube.)
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To see an object, its light rays have to meet on the retina in the focal point.
That's not what focus is. Not that the light rays from A and B don't meet up at the same point on the retina.
If all the light from an object was brought together at a single point, there would be no image formed. Lenses don't do that. Instead they bring light from different points on the object together at different points on the retina.
It is impossible for any lens to bring the light from different points together at single point. To see this just imagine the lens in reverse.
This is true of any lens. Even if you have used a "burning glass" to focus the sun, you don't bring the sun's rays to a single point; you create a small image of the sun, small enough to be very hot.
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$\begingroup$ It is impossible for any lens to bring the light from different points together at single point. If I consider a vertical plane between the object to the left and the eyeball to the right, then this plane intersects, for example, the three blue rays in three points. Image I remove the object but put three glowing objects at these three points. Then they emit rays along the blue lines and these three rays all meet at point $a$ on the retina. So, light from three different points is brought together at the point $a$, right? So, what do you mean when you say this is not possible? $\endgroup$– StefanHMar 27 at 13:32
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$\begingroup$ Consider putting a point source at that focal point, then the light rays of light emitted from that point source would follow exactly the same path backwards, ultimately forming an image of a tree. So how did the lens make an image of a tree from a point source? why not an image of a dog? The image forming process must be reversible, so the image cannot be a single point. $\endgroup$– James KMar 27 at 17:41
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$\begingroup$ You mean, in one vertical plane? Otherwise, I guess, it is possible. For example, move the point $B$ up and to the right to the intersection point of the uppermost red line and the lowermost blue line. Then, the rays of the replaced point $B'$ hit the retina at the point $a$ as well. If this is right, then the information that arrives at the point $a$ does not correspond to a single point. I am not quite sure why our eye is not affected by such obstructions and still can reconstruct the image perfectly, but according to this explanation some uncertainty in $a$ is present. $\endgroup$– StefanHMar 28 at 13:20
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$\begingroup$ No, if you move point B to B' as you say, then point B will not be in focus on the plane of the retina $\endgroup$– James KMar 28 at 17:18
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There might be a confusion about what you call focal point.
The focal point of an ideal lens is a single given point. Only the light rays parallel to the optical axis will pass through the focal point. Parallel rays at an another angle will be collected at different points in the so-called focal plane.
Light rays coming from (not too distant) point sources will be collected at other points again, behind the focal plane. (However the lens in our eyes can adapt and reduce its focal distance, so that the image formed behind the focal plane falls on the retina.)
