What Is the Radius of Curvature in Optical Applications?

[miltzi]

What exactly is the radius of curvature of an object? And how would this be applied to a question such as the following:

A glass porthole of a submerged craft has parallel curved sides, both of radius of curvature R. What would R be in order that an object in the water 2m away from the porthole should appear undisplaced to an observer inside the craft? Refractive index of water: 4/3, glass: 3/2, air: 1

 

[Drakkith]

Does this help:http://en.wikipedia.org/wiki/Radius_of_curvature_(optics )

Basically its the radius of the circle the curve of the surface would make.