# Homework Help: Range of Powers of Eye Lens - Optics Question

1. Jan 31, 2010

### Keano16

1. The problem statement, all variables and given/known data

The human eyeball is approximately spherical with a diameter of around 25mm. At the back of the eye is the retina, while at the front is a compound lens comprising the cornea (of fixed focal length) and the lens itself (with variable focal length), which can be approximated as a thin lens at a distance of 17mm from the retina. The eye is capable of focusing on objects at distances varying from the near point (about 25cm) to infinity. Estimate the range of lens powers which can be achieved by the lens and the cornea in combination.

2. Relevant equations

Power = 1/f
1/u + 1/v = 1/f

3. The attempt at a solution

I was wondering if we could get the expression for 1/f for each one of the cornea and the lens, and them add them and take the reciprocal to find the total power, though I am not sure whether it applies to this case?

As far as the range is concerned, I presume it basically entails taking the object distance to be at infinity and at the near point (25cm)

Any help would be great.

2. Jan 31, 2010

### Stonebridge

With the object at infinity you get f (and the power of the lens combination) directly from the retinal distance.
The other extreme is with the object at 25cm. The focal length (and power) is found in this case using the lens formula you have quoted.
This gives the range of powers.