Geometrical optics

Two rays traveling parallel to the principal axis strike a large Plano-convex lens having a refractive index of 1.60 (ray strike plane surface first).

If the convex face is spherical, a ray near the edge does not pass through the focal point (spherical aberration occurs). Assume this face has a radius of curvature of 20.0 cm and the two rays are at distances x = 0.500 cm and y = 12.0 cm from the principal axis. Find the difference (in cm) in the positions where each crosses the principal axis.

Give answer up to 2 decimal places.