How does the shape of a lens affect focal lengths?

[jst81161]

Homework Statement

Why do two outer rays converge to a different focal point than the inner rays?

Homework Equations

The Attempt at a Solution

The distance to the focal point of the inner rays is shorter than the outer rays, the light has to bend a lot more for the outer rays

 

[Kurdt]

I don't think you've really hit the nail on the head. Think about the ideal shape of a lens and the actual shape of a lens.