Why does someone with normal vision not need glasses?

[Xelb]

Homework Statement

These are more-so explanation based and there is no math involved.

1) How come someone with "normal vision" does not require glasses?
2) Why does a far sighted person require glasses with a positive focal length?
3) Why does a near sighted person require glasses with a negative focal length?

It's bad enough that we have not covered the chapters dealing with optics in lecture yet, so some brief clarification would be greatly appreciated. Thank you!

 

[berkeman]

Homework Statement

These are more-so explanation based and there is no math involved.

1) How come someone with "normal vision" does not require glasses?
2) Why does a far sighted person require glasses with a positive focal length?
3) Why does a near sighted person require glasses with a negative focal length?

It's bad enough that we have not covered the chapters dealing with optics in lecture yet, so some brief clarification would be greatly appreciated. Thank you!

Welcome to the PF.

We require that you show some effort in answering these schoolwork questions before we can offer much tutorial help. That means that if you have not covered this in your coursework yet, you should use the web or your textbook to do some reading about the basics involved. Then you should be able to take at least a afirst cut at answering the questions, and we can check your answers for you.

I did a Google search on Corrective Lenses Conceptual Questions, and this was one of the first hits (Google suggested the last two words of the search):

http://www.angelfire.com/scifi2/eyeglass/

That should help you out...

 

[Xelb]

So I was able to figure out the answers to questions 2 and 3.

For 1, however, I'm still not sure about it...but here's my guess:

Because a normal eye is neither elongated nor smaller, this means that neither a converging or diverging lens is needed in order to focus nearby objects on the retinal surface. On a normal eye, the image is produced normally on the retina because the refraction process begins when light reaches the eye.

 

[berkeman]

So I was able to figure out the answers to questions 2 and 3.

For 1, however, I'm still not sure about it...but here's my guess:

Because a normal eye is neither elongated nor smaller, this means that neither a converging or diverging lens is needed in order to focus nearby objects on the retinal surface. On a normal eye, the image is produced normally on the retina because the refraction process begins when light reaches the eye.

Sounds reasonable to me.