Difference between revisions of Eyeballs
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extend the order-of-magnitude calculations a little. Probably complete rubbish, but what can you expect from a physicist completely clueless about biology.
Viceroy Sam (talk | contribs) m (→Focusing Power: General fixes, typos fixed: For example → For example,) |
(extend the order-of-magnitude calculations a little. Probably complete rubbish, but what can you expect from a physicist completely clueless about biology.) |
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* '''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 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. | * '''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 == | == 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. | 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. | ||
== [[wikipedia:back of the envelope|Back-of-the-envelope]] calculations == | |||
We can use simple optics, the thin lens equation <math>\frac{1}{x} + \frac{1}{y} = \frac{1}{f}</math> and some very approximate numbers to give order-of-magnitude estimates of some of the quanties 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== | ==See Also== | ||