So I have a problem where i am getting confused what happens when an object is placed in front of a myopic eye. Also I have another problem ie. why the image becomes blurry if it is formed beyond the focus of any convex lens(According to my experience). What I actually mean is why the image is blurry after the F1 of a Convex Lens(I think the same happens in the case of a myopic eye where the image is although formed but it is blurry so that the viewer cannot recognize it).The image is formed by joining the two rays from the optical centre and an image is formed.
Similarly I have the final problem — what happens if we place an object in front of an eye(normal basically) similar to that when we place it infront of a general convex lens.
You can understans what I am telling by this example.
Here the object is placed in the middle of the lens so that both the rays from its head and bottom are taken and convered to form an image!
But here the object is palced slightly upward on the principal axis! This results to an inverted images in real cases(mostly ignoring the virtual cases)
Thus what happens when we do a similar approach for a human eye.
Summerizing my question:
- Difference between normal eye and myopic eye with ray digrams to instruct what actually happens?
- Why an image becomes blurry when it formed by joining rays taken from optical centre at the retina or screen.
- Pls tell the case where the object is placed slightly above the principal axis in front of our eye.
Hope the readers may understand my questions!
Best Answer
Brief summary: the difference between a normal eye and a myopic eye is in how they bend light coming from a point source that is very far away ("at infinity") when the muscles that change the shape of the lens are in their completely relaxed state. A myopic eye bends light from a point source that is very far away "too much" so that the rays meet in front of the retina rather than exactly at the retina.
Part 1: What it means for an eye to be focused on an object
Consider the following picture:
The eye is focusing on the green object, because the lens is flexed in just the right way that all rays emanating from the bottom of the object converge to a single point on the retina, and all rays from the top of the object converge to another single point on the retina. This must be true for all points of the object, that all rays from that point are bent by the lens to reach a unique single point on the retina. In this way, each point on the retina corresponds to a unique point in the (two-dimensional) space in front of the lens so that the brain can interpret "these molecules at this point on the retina fire" as "there is a point source of this color at this point in space".
Part 2: What it means for an eye to be unfocused
Consider the following picture, which corresponds to a too-relaxed eye:
The rays emanating from a point source hit the lens are bent in such a way that they meet behind the retina. According to the diagram, we can see that multiple points on the retina are hit by the different rays, and hence the brain is forced to interpret this object as being smeared out in space, because each point the retina is supposed to correspond to a different point the visual field in front of the lens. Furthermore, according to this diagram,
we can see that there can be multiple point sources out there in the field of vision whose light rays converge on the same point on the retina, and effectively the brain must smear these two points into one.
If instead the muscles in the lens are too flexed, leading to the rays being bent too much, the following occurs:
We can see that light rays from the object that are farther away are bent too much, so that they meet at a point in front of the retina, leading to many points on the retina being activated by these multiple rays and hence the image of the point source begin smeared out on the retina, meaning that the brain must again interpret this light as coming from different points in the visual field. In contrast, an object that is nearer can be in focus, as shown in the diagram: because these rays are coming in at a wider angle, the lens needs to bend them more in order for them to meet correctly at the retina.
Part 3: Focusing on an object that is "far away"
Consider the following diagram.
Light rays coming in from a point source "very far" away come in essentially parallel to each other, hit the lens and are bent so that they meet at the retina. This is what happens in a "normal" eye in the completely relaxed state; that is, the muscles that change the shape of the lens are completely relaxed. Any flexing of these muscles causes the lens to bulge out, and hence the light hitting the lens is bent more when the muscles are not completely relaxed. A point of terminology: the farthest point away on which an eye can focus is called the far point of the eye. For "normal" eyes, this point is essentially infinitely far away (at least to the extent that we can treat the light rays coming from this point as coming in parallel to each other).
Part 4: Myopia
A myopic eye is one that has a far point which is closer than infinity. That is, there some distance $d_{\textrm{far}}$ from the lens beyond which the lens can't create an image in the retina. Thinking mechanically, for an eye that is myopic, the rays that come from far away are bent too much by the lens in the completely relaxed state. There is point a maximal distance away for which the light rays emanating from this point are bent by the lens (in it's completely relaxed state) to meet at the retina. This point is called the far point. The issue is then illustrated by one of the previous pictures, included again here:
Suppose the point labeled "In Focus" is the far point of the lens. Then any light rays coming in "shallower" than those coming from the far point will bend in such a way that they meet in front of the retina; for instance, the light rays coming from the point labeled "Out of Focus" will meet in front of the retina as shown (and we've already explained why this corresponds to the point source being "out of focus").
Part 5: Correcting myopia
I think it helps the understanding of nearsightedness to explain how we use corrective lenses (spectacles or contact lenses) to correct it. Consider the following (busy) diagram:
The purpose of a corrective lens (labeled "lens axis") is to make it so that the eye in its completely relaxed state can focus the rays coming from a point source far away on a single point on the retina. To do this, the lens is designed in such a way that it takes rays coming in parallel and bends them so that they *appear as if they're coming from the far point". Let's explain this using the diagram.
The point labeled FP is the far point of the eye.
The blue dashed line is what occurs without the corrective lens in place: In the completely relaxed state, the lens of the eye will bend light rays coming from this point in such a way that they meet at the retina (Remember, this is the definition of the far point.)
The red dashed line is what occurs without the corrective lens in place: In the completely relaxed state, light rays coming from far away (solid black) are bent at the lens of the eye so that they meet in front of the retina. (Remember, this is what it means to by myopic and is related to the definition of the far point.)
The solid black line from left all the way to the retina is what occurs when the corrective lens is in place: In front of the eye, we place a corrective lens that is designed to take light rays that are coming in parallel and bend them so that they lie along the lines coming directly from the far point. These rays are naturally bent by the eye lens (in its relaxed state) in such a way that they meet at the retina. Thus, the light rays from a point source at "infinity" follow the solid black line in the diagram, meeting at a single point on the retina, and hence the brain is able to interpret that as light coming from a single point in its visual field!