index of refraction

[harry a saake]

Since according to the AO handbook, the four mediums light passes through,cornea, crystalline lens, aqueous humor, vitreous humor can all have a different index and cause a refactive error,how much can it cause, does one see through the sum total of all the indexes or just one of them and if so which one, and in theory any way if the rx was caused by the index only, could we possibly fit different index lenses and correct the rx?

[Jackie L]

Interesting theory, Harry. Knowing that we see with our brain and that our eyeballs are the tools that we see with, would our brain be able to handle different index lenses place before the eyes?

Jackie O

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Still a Maina for now

[shanbaum]

Originally posted by Jackie O:
Interesting theory, Harry. Knowing that we see with our brain and that our eyeballs are the tools that we see with, would our brain be able to handle different index lenses place before the eyes?

Jackie O

I'm sure if the index of the various parts - especially the crystalline lens and the cornea - could somehow be "adjusted", refractive errors could be corrected thereby. Tall order. But I don't think the brain would object, at least, I don't think mine would.

[chip anderson]

Harry, About 20 years ago I heard a news report that the Japanese (I probably get in trouble with Steve again mentioning the Japanese) had plans to market a pair of glasses which had an electronicly controlled index. The object was you could dial up the index and correct for any amount of presbyopia. Have not seen or heard a word about this since.

Have a good one.

Chip

[Dave Nelson]

Harry, thats a bit like calling the cornea to "steep" in a myope. A steep cornea can cause a refractive error, like myopia- but only if the axial length of the eye is to long as well. A person with a short axial length might be hyperopic.. unless they have a correspondingly steep cornea. Although myopia is sometimes refferred to as "axial" myopia, meaning the length of the eyeball is to long, it still would'nt be a problem if the cornea was really flat. (lasik=flatter cornea for myopes.) So when components of the eye get "out of synch" (like the index) a refractive error results, but you cant really point to one part of the eye as the cause.

[dbracer]

Since according to the AO handbook, the four mediums light passes through,cornea, crystalline lens, aqueous humor, vitreous humor can all have a different index and cause a refactive error,how much can it cause, does one see through the sum total of all the indexes or just one of them and if so which one, and in theory any way if the rx was caused by the index only, could we possibly fit different index lenses and correct the rx?

Actually it is easiest to think of the air as being a rare media. Index? "1." The eye a dense media. Index? Depends on the eye which is to say I can't remember the average.

A relative prospective is that a vacuum actually has and index of exactly "1." Air? 1.00022. 1 is close enough, for cryin' out loud. Water? 1.33.

The index of the dense media can be calculated by finding the change in the vergence of the light as is passes from the rare media to the dense media with respect to the curvature of the front of the eye. This provides a resultant index of the entire eye as opposed to trying to consider the various minute changes of media from cornea to aqueous to lens etc. It's a physics nightmare.

As someone has already said we can think of the eye as being too long for itself, or it has too much power for its length - which is saying the same thing.

But one might ask, "Is there a way of determining whether the eyeball is actually to long, or whether it has aberrant power?

Actually that has been determined. We usually think those eyes more than 6.00 diopters myopic are more likely of an axial length problem, and those eyes less than that are thought to have aberrant power.

Most of that work on corpses was published back in the mid-60's, or even earlier.

Interesting stuff.

Respectfully,
dbracer

[Snitgirl]

dbracer,

Welcome to Optiboard Doctor...:cheers::cheers:

[dbracer]

dbracer,

Welcome to Optiboard Doctor...:cheers::cheers:

Snitgirl,

Thank you for your kind welcome. I envy your avatar.

Respectfully,
dbracer

[HarryChiling]

could we possibly fit different index lenses and correct the rx?

Shanbaum pointed out an interesting thing a few months back that stumped me, it's that a plano lens woud technically be considered a plus lens since the back curve is flatter than the front curve and that a two concentric curves create a lens that has power.

So with this knowledge I woud say yes it would be possible to fit lenses that don't use curves to create power, but use the same curves and different indexes insted to change the Rx. Would be a whole lot difficult considering it is a heck of a lot easier to vary curves mechanically than to search for all the varying indexs that would create all the varying prescriptions that we see. If I can find the article their is an interestign topic on GRIN or GRadient INdex lenses. The paper basically dealt with progressives and varying the power using a gradient index, by the way this is the technology that Ophthonix (probably butchered the spelling) supposedly uses with the wafer in the center of their lenses.

[chip anderson]

I often mix indexes to achive matching edge thicknesses on minus imballances and occasionally with plus imballances. Been doing this about 20 years without any problems from same. Would not switch progressive designs or brands for this though.

Chip

[Darryl Meister]

Shanbaum pointed out an interesting thing a few months back that stumped me, it's that a plano lens woud technically be considered a plus lens since the back curve is flatter than the front curve and that a two concentric curves create a lens that has power.

I think you mean that the back curve is slightly steeper than the front curve. What Robert probably meant (without hunting down the thread in question) is that a lens with equal front and back curves would actually be a plus lens because of the gain in vertex power at the plane of the back surface caused by the thickness and curvature of the lens.

does one see through the sum total of all the indexes or just one of them and if so which one, and in theory any way if the rx was caused by the index only, could we possibly fit different index lenses and correct the rx

The eye actually sees through all four media, though the net result can be approximated with a single equivalent surface and index. The so-called "reduced" eye, for instance, is represented with a single refracting surface having a radius of curvature of 5.56 mm and a refractive index of 1.333 (close to the refractive index of the vitreous and aqueous humors).

As dbracer suggested though, a change in the refractive index of a medium surrounding the eye will only affect the power at the front surface of the cornea (though refraction at the other surfaces would need to be adjusted accordingly as light propagates through the eyeball). Also, while we're discussing technical minutiae, the refractive index for lenses and similar optical elements bounded by air is actually the relative refractive index. This is the ratio of the velocity of light in air to the velocity of light in the medium, which is subtly different than the absolute refractive index of a medium in a vacuum.

Further, as Harry noted, you can use similar curves with a different refractive index to produce a different power. Similarly, you can use different curves and a different refractive index to produce the same power. This is exactly why high-index lenses are thinner than traditional lenses; a higher-index material allows the use of flatter, thinner curves for the same power.