What is the focal length of the lens?

Question

Can someone please help me with this question?

What is the focal length of the lens in your eye when you read a book that is 35.0 cm from your eye? The distance from the lens to the retina is 0.19 mm.

Thanks!

Answer 1

The idea here seems to be to use the thin lens equation (ref.) even though it's not especially suited to the eye, since a lot of the focusing power of the eye is not in the lens but in the cornea, which you can see by noting the curvature of the eye's front surface. Anyway, the thin lens equation says

f = 1/(1/do+1/di) = 1.802 cm, assuming you meant 19, not 0.19 mm from eye to retina.

Answer 2

The focal length of the eye is about 17 mm.

this is the distance from the lens to the retina.

That's why in image is in focus on the surface of the retina (the center anyway)

bye for now.