Optics & Vision: Understanding the Eye

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In summary, the eye forms a real and inverted image on the retina by converging the light rays from an object onto a specific point. This is achieved through the manipulation of the lens's power and involves complex image processing within the retinal layers. The process can be compared to holding a lens up to a window and getting a real image on a sheet of paper on the other side. However, some diagrams and videos may be misleading in showing the convergence of rays before the retina, which could lead to misconceptions about nearsightedness.
  • #1
esmeralda4
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Hi.

I'm new to this forum and wondered if anyone can help with this problem I have understanding how vision works.

In some ray diagrams of the eye it show the rays converging to a point on the retina and on some diagrams it shows them converging before the retina and arriving at the retina inverted.

The second one makes more sense to me as I imagine that to be the inverted image being formed on the retina. (much the same as a lens forming an image on a piece of paper).

However the other diagram appears on many website and textbooks too.. Surely the rays of light are not converged into a single point on the retina? How would the retina and optic nerve make sense of this??

Can anyone help with my mis conceptions?!

Thanks
 
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  • #3
Thanks.

So does that mean the websites that show parrallel rays from infinity converging to a single point on the retina are wrong??
 
  • #4
No, but the light from some object is not going to be parallel. Unless it is infinitely far away. (And then you can't see it!) If it is far enough away, it will look like a little dot anyway.
 
  • #5
esmeralda4 said:
Hi.

I'm new to this forum and wondered if anyone can help with this problem I have understanding how vision works.

In some ray diagrams of the eye it show the rays converging to a point on the retina and on some diagrams it shows them converging before the retina and arriving at the retina inverted.

The second one makes more sense to me as I imagine that to be the inverted image being formed on the retina. (much the same as a lens forming an image on a piece of paper).

However the other diagram appears on many website and textbooks too.. Surely the rays of light are not converged into a single point on the retina? How would the retina and optic nerve make sense of this??

Can anyone help with my mis conceptions?!

Thanks

Oof- lots of stuff here. It just goes to show that the eye is much more complex than is usually discussed in optics classes.

A few preliminaries: there are focal planes and pupil planes. Pupil planes are Fourier transforms of focal planes.

So, for a relaxed eye focused at infinity, an object point corresponds to a plane wave at the pupil plane (typically located near the lens), which is then focused to a point at the retina (neglecting aberrations). When we focus on an object that is closer than infinity, by manipulating the power of the lens within our eye, the image is formed on the retina (for normal or corrected vision). This explains both diagrams.

To be sure, there is substantial image processing within the retinal layers: edge enhancement, motion detection, local averaging, all kinds of stuff. The actual image projected onto a retina looks horribly aberrated.

For all this and much more, see "Optics of the Human Eye" (Atchison and Smith)
 
  • #6
esmeralda4 said:
In some ray diagrams of the eye it show the rays converging to a point on the retina and on some diagrams it shows them converging before the retina and arriving at the retina inverted.

If the second diagram shows light rays converging to a point in the middle of the eyeball, then diverging to hit the retina, that's wrong, at least for an eye that's actually focused on the object. Each individual point on the object produces a bundle of light rays that pass through the lens and are focused to an image point on the retina. The rays diverge as they enter the lens, and the lens converges them towards the image point. Different object points produce different image points. The overall image is inverted, but the inversion effectively happens at the lens, not in the middle of the eyeball.
 
  • #7
Thanks for your help everyone. So you could draw a diagram showing parallel rays coming from different points on a object or many rays coming from a single point on an object in all direction? Is this the same as when you hold a lense up to a window and get a real image formed on a sheet of paper the other side of the lense?
 
  • #8
esmeralda4 said:
Is this the same as when you hold a lense up to a window and get a real image formed on a sheet of paper the other side of the lense?

Yes, the image formed by the lens of the eye on the retina is a real image just like the image on a sheet of paper in your example.
 
  • #9
Ok, sorry to keep going on about this but i think I've got it.

Am I right then in saying that this video clip on you tube is incorrect when it talks about pararrel rays entering the eye?

It shows the rays converging before the retina. Surely that would mean the person is short sighted?

http://www.youtube.com/watch?v=15P8q35vNHw&feature=related
 
  • #10
I'd say that the video clip is misleading when it shows rays converging before the retina. (What I think they are actually showing are principal rays used to determine where the image is focused. That criss-cross point is not the image. Confusing, eh?)

The image is focused on the retina, except as you noted in the case of nearsightedness (or farsightedness).

You might find this a more informative discussion of the eye and vision: http://www.glenbrook.k12.il.us/gbssci/Phys/Class/refrn/u14l6b.html" [Broken]

Here's a good picture of how a lens (like your eye) focuses light to form a real (and inverted) image: http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/image.html#c1"
 
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  • #11
jtbell said:
Yes, the image formed by the lens of the eye on the retina is a real image just like the image on a sheet of paper in your example.

Ophthalamology students have been known to carefully scrape the back off a sheep's eyeball to see that image.
 
  • #12
Thank you.

Sorry, last question on this I promise!

If I hold a converging lens to some writing and move the lens gradually nearer to the page until the writting appears in focus is the distance now between the lens and the paper the focal length?
 
  • #13
No, becasue of the refractive power (and accomodation ability) of your eye. The quick and dirty method of determining the focal length of a lens is to image a light bulb (ceiling fluorescents are great) onto the floor; the source is effectively at infinity, so the distance from lens to floor is the focal length. YMMV.
 

1. What is the purpose of the eye?

The eye is a sensory organ responsible for detecting light and converting it into electrical signals that are sent to the brain for processing. Its primary function is to enable us to see and perceive our surroundings.

2. How does the eye focus light?

The eye uses a process called accommodation to focus light. The lens in the eye changes shape to adjust its focal length, allowing it to focus light from objects at different distances onto the retina.

3. What is the role of the retina in vision?

The retina is a layer of tissue at the back of the eye that contains specialized cells called photoreceptors. These cells are responsible for converting light into electrical signals that are sent to the brain via the optic nerve. The retina is essential for vision as it is where the initial processing of visual information occurs.

4. How do we perceive color?

The retina contains three types of photoreceptors, each sensitive to different wavelengths of light - red, green, and blue. These photoreceptors work together to enable us to perceive a wide range of colors. The brain then interprets these signals to create the perception of color in our vision.

5. What is the difference between nearsightedness and farsightedness?

Nearsightedness, also known as myopia, is a condition where distant objects appear blurry, while close objects are clear. This is caused by the eye being too long or the cornea being too curved, resulting in light being focused in front of the retina.Farsightedness, or hyperopia, is the opposite, where close objects appear blurry, and distant objects are clear. This is usually due to the eye being too short or the cornea being too flat, resulting in light being focused behind the retina. Both conditions can be corrected with glasses, contact lenses, or surgery.

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