How Do You Calculate the Change in Focal Length for a Human Eye Lens?

[Ryo124]

Homework Statement

Suppose the lens in a human eye has far focal length equal to f0 = .0405 m, for distant objects to be in focus.

(a) An object K is 1.72 m from the lens. How much should the focal length change to focus on the object K?

(b) What is the magnification ( = image size divided by object size) for the object K?

Homework Equations

1/do + 1/di = 1/f

The Attempt at a Solution

I figured the image distance (di) to be .0415 m, but I'm unsure of how much the focal length should change to focus on the object K.

I tried subtracting the object distance from the focal length (vice versa), doesn't work.
I tried subtracting the image distance from the focal length (vice versa), didn't work.
I tried subtracting the object distance from the image distance (vice versa), incorrect.

Also, di/do is not giving me the correct magnification, it is not .241.

Someone help please. It looks like I'm going to miss my deadline but I still need to know how to solve this type of problem for the test.

 

[mgb_phys]

The simple lens formula is correct 1/object + 1/image = 1/f
With the image at infinity object=infinity so 1/image = 1/f
If we assume that the eye doesn't change size and the lens doesn't move,then image distance remains the same and equals the focal length for the infinite object ie 0.0405m

The we simply put in the new object distance.
1/1.72 + 1/0.0405 = 1/f , giving f = 0.0396
So the change is simply 0.0405-0.0396 = 0.001 ( 0r -0.001, focal length gets less )

 

[Ryo124]

Thanks mgb for clearing up my misunderstanding. It makes sense now.