Myopia

[chiddler]

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Imagine an eyeball that is too long, or a myopic individual. Why can they only see nearby objects? If the light rays are converging too early, shouldn't the person be blind?

ie, why/how does changing the shape of the lens in a myopic individual allow sight of nearby objects but not far?

 

[MedPR]

Imagine an eyeball that is too long, or a myopic individual. Why can they only see nearby objects? If the light rays are converging too early, shouldn't the person be blind?

ie, why/how does changing the shape of the lens in a myopic individual allow sight of nearby objects but not far?

This is how my anatomy teacher explained it to us last semester. Nearsighted people over-refract the light, thus shortening the focal length and making all the rays converge at some point in front of the macula. Think about how the light rays diverge away from the cornea (not towards the retina, but back out into the world/object you are looking at. I know this is kind of backwards, but bear with me). Notice that as you get further and further from the cornea, the height of the light gets greater and greater? So if you consider only the very bottom ray and the very top ray that are coming into the eye, you can see that they are already angled in a similar fashion to what the cornea wants to do with it (they are pointing towards the focal point). Combined with the fact that a myopic eye over-refracts, these rays are bound to focus somewhere in front of the macula.

If you think about how rays of light reflect off of a very nearby object, you can see that they are, more or less, directly parallel to the ground and thus not angled at all, which means that even though your myopic eye is over-refracting, it can't over-refract the nearby object enough because the rays are not pre-bent as they are for a far away object.

That's why you use a diverging lens for myopic individuals. Light hits the lens and diverges towards the floor and ceiling (away from midline) so when it hits your over-refracting cornea the net effect is just-right-refraction.

It's not a physics explanation by any stretch, but it made sense to me :/

Edit: Also, I realize I didn't really answer the question regarding the type of myopia you are talking about. The myopia I'm describing is not related to eye length. If the eye is too long, the retina is farther from the cornea and lens than usual. The word "long" in this context is referring to anterior to posterior length, not inferior to superior length. In other words, the eye is too deep, not too tall. You can probably easily see how extending the eye without changing the lens or cornea can cause the light to be focused too soon. You're not blind because you don't actually have to focus light onto the retina in order to see it. Physiologically the goal is to focus light on a region anatomically called the macula, which is the region of greatest visual acuity. You have photoreceptors throughout most of your eye though, so light anywhere will produce some kind of image.

 

[chiddler]

"Notice that as you get further and further from the cornea, the height of the light gets greater and greater"

Can you explain here, please. Further from the cornea - you mean going deeper inside the eye or going outside of the eye further and further?

And with whichever one it is, could you describe a bit more?

thanks for the response.

it wasn't important for me the type of myopia; i wrote it to help with visualization. but rather, my focus was the question I wrote. Namely, how is nearsightedness possible, given that light rays always converge too close to the eye?

 

[MedPR]

"Notice that as you get further and further from the cornea, the height of the light gets greater and greater"

Can you explain here, please. Further from the cornea - you mean going deeper inside the eye or going outside of the eye further and further?

And with whichever one it is, could you describe a bit more?

thanks for the response.

it wasn't important for me the type of myopia; i wrote it to help with visualization. but rather, my focus was the question I wrote. Namely, how is nearsightedness possible, given that light rays always converge too close to the eye?

Further away from the outside of the eye.

See how as you look at a further away object, the light comes in from a higher/lower point than if you were to look at something very near your cornea?

 

[chiddler]

oh! this makes a lot of sense now thank you.

 

[pfaction]

Does the cornea or the lens refract more? I've read it was the cornea.

 

[MedPR]

Does the cornea or the lens refract more? I've read it was the cornea.

Cornea does. Myopia can occur with abnormalities of the eye, cornea, or lens though.

 

[pfaction]

Good, thanks. You're doing really well man. April 5th, right? GL...I'm really nervous about Apr 28th, I finished all the material from TPR now just going through it all again before my next AAMC. Verbal, holy ****, I just suck at it.

 

[Class1P]

I thought I understood this until just now.


oh wait, it just clicked again.

This makes sense. The further you get from the focal point of the outside of the eye the smaller and closer to the internal focal point of your eye the image becomes.


Object----r------------f---------()----------f---image-------r-------------- )retina

so the further your object is the more distant the image focus is from your retina. which means there is more distortion and size increase

if your object is close to the focal point of your eye on the outside:

--------r---------object---f---------()----------f--------------r-------------Object- )retina

the image is closer to the retina and larger and inverted.