Difference between revisions of Eyeballs

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== [[wikipedia:back of the envelope|Back-of-the-envelope]] calculations ==
 
== [[wikipedia:back of the envelope|Back-of-the-envelope]] calculations ==
  
We can use [[Optics related math]] and some very approximate numbers to give order-of-magnitude estimates of some of the quanties involved.
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We can use [[Optics related math]] and some very approximate numbers to give order-of-magnitude estimates of some of the quantities involved.
  
 
To estimate the focusing power of an [[emmetropic eye]], we might take the [[axial length]] as around 2.5cm. For [[distance vision]] (parallel incident light) that number is simply the [[focal length]] of the eye at rest, giving 40 [[Diopters]]. If we take the [[near point]] as about 25cm, that requires an additional 4 dpt of focusing power from the lens.
 
To estimate the focusing power of an [[emmetropic eye]], we might take the [[axial length]] as around 2.5cm. For [[distance vision]] (parallel incident light) that number is simply the [[focal length]] of the eye at rest, giving 40 [[Diopters]]. If we take the [[near point]] as about 25cm, that requires an additional 4 dpt of focusing power from the lens.

Revision as of 11:40, 31 July 2020

Parts of the eye

Schematic diagram of the human eye en
  • Sclera - The white of the eye
  • Cornea is the clear outer layer of the eye through which you can see the Iris and Pupil. It has an Index of Refraction of 1.376[1], and a curved outer surface, contributing to the refractive state of the eye. The Cornea provides about 80% of the eye's total refracting power. If you have LASIK or PRK surgery it thins the cornea to change your refractive state.
  • Aqueous humor - the fluid supporting the cornea
  • Pupil - the hole where light enters the eye
  • Iris - the Iris is the colored part of the eyeball that contains the muscles that control the opening size of the pupil.
  • Ciliary muscle is a ring of muscle fibers in the eye that control the tendons supporting the natural lens of the eye, and controls the flow of aqueous humor behind the cornea. The Ciliary muscle is controlled by the Ciliary ganglion, which is a complex intersection of several nerve systems. The action of the ciliary muscle is the primary source of accommodation and ciliary spasm which causes pseudomyopia.
  • Lens - The part that changes the focus distance of the eye
  • Rods and cones - Rods and Cones are the sensory cells in the back of your eye that detect light.
    • Rods sense only light intensity, not color. They require lower levels of light to trigger, and so work better in low-light conditions. They are more sensitive to movement, and tend to be concentrated on the periphery of the retina. If you are outside at dusk, you may feel a sudden switch of your vision from color vision to black and white, this is your visual cortex switching to only rod input when cone input isn't working as well in dim light.
    • Cones are the cells that detect color in your eye, but require much higher light levels to trigger. They concentrated in the macula, where high-resolution acuity is required. There are three different types of cones that respond most strongly to three different wavelengths of light, though there is a large overlap. Your visual cortex takes the combined response of the three types of cones and makes up the blended color in your mind. Magenta for example is an imaginary color. It's the color your brain makes up to explain why both short and long wavelengths of light are detected, but not the wavelengths in the middle. Most colors are on the color spectrum you learned in school (Red, Orange, Yellow, Green, Blue, Indigo, Violet), and will trigger a single cone type, or two adjacent cone types.
  • Retina - the tissue that supports the rods and cones.
    • Macula - a small area of the retina with a higher density of light receptors.
    • Fovea - a tiny pit in the macula with the highest density of cones, for highest resolution vision.
  • Choroid - the structure behind the sclera. It can change thickness (on a timescale of days) to make small adjustments to the axial length
  • Vitreous humor is the clear gel filling the majority of the eyeball. It is where true floaters live. This gel is important for helping the eye hold its shape and maintain the correct pressures inside the eye even when air pressure changes. In adults, the gel has a complex structure, with different thicknesses in different parts.
  • Vitreous detachment is attached to the outer wall of the eye in multiple locations, but can become separated. When separated from the retina it does not support the retina fully, and puts you at higher risk for retinal detachment. It can also leave behind a large floater that impairs vision.

Axial Length

The primary cause of differences in refractive state is the length of the eye, referred to as axial length, relative to the focusing power. Long eyeballs are associated with myopia, as the natural lens of the eye, even when fully relaxed, focuses light too far forward of the retina.

Back-of-the-envelope calculations

We can use Optics related math and some very approximate numbers to give order-of-magnitude estimates of some of the quantities involved.

To estimate the focusing power of an emmetropic eye, we might take the axial length as around 2.5cm. For distance vision (parallel incident light) that number is simply the focal length of the eye at rest, giving 40 Diopters. If we take the near point as about 25cm, that requires an additional 4 dpt of focusing power from the lens.

If we now suppose that myopia is due entirely to elongation (ie the focusing power is unchanged), how much does the axial length need to increase to bring the blur horizon to 40cm ? With a 40 dpt lens and a source object at 40cm, the image would form 26.67mm from the lens, giving an estimate of elongation of 1.67mm or 6%.

In the same way, we can calculate the new near point : with a lens of 44dpt and an image location of 26.67mm, the source object would be at around 15cm.

See Also

References

  1. Nave, R (2020-05-25). "Scale Model of Eye". HyperPhysics.