Focal Length of Eye Homework: C, F Calcs & Myopia/Hyperopia

[ahrog]

Homework Statement

The diagram shows the eye of a person with normal vision looking at an object that is very far away.
a) Determine the focal length of this persons eye.
b) Later in this person's life, the focal length will change to between 2.1 cm and 2.3 cm. Will this person suffer from myopia or hyperopia? Explain your answer.
c) Will this person require a convex or concave lens to correct this problem?

Homework Equations

Center of curvature= 1/2 diameter
Focal length= 1/2center of curvature

The Attempt at a Solution

a) C=1/2D
=1/2(2.5cm)
=1.25cm

F=1/2C
=1/2 (1.25)
=0.625

It doesn't make sense though, that focal length seems too small/odd. It says in the question that it changes to between 2.1 and 2.3, so my answer seems wayyyyy off...http://img23.imageshack.us/img23/9751/eyecopyw.jpg [Broken]

 

[rl.bhat]

Focal length= 1/2center of curvature
This is true for mirrors, not for lens.
In the above problem f = 2.5 cm.

 

[ahrog]

Focal Length of 2.5? Wow, that seems...too simple?

 

[rl.bhat]

When a parallel beam of light falls on a lens , it forms an image at focal point.

 

[ahrog]

Thanks. Another question though. How would you find the distance in which an object can be seen clearly from by just using the number 2.5 (and possible 2.1 and 2.3)?

 

[rl.bhat]

Using f as 2.5 cm and di as 2.1 and 2.3, find do. If do is lass than 25 cm,convex lens to be used. And if it is more than 25 cm concave lens to be used. For a healthy eye the object should be at 25 cm to see distinctly.