f is the function of the upper surafce of the lens
g is the function of the lower surface of the lens
ff and gg are the derivatives of both functions
x and y are the coordinates of the light source (the starting point of the ray).
(xo,f(xo)) gives the Point where the Ray meets the upper surface of the lens. It would be enough for me if it would be possible somehow to make a table of this xo's. So that several rays meet the lens at the upper surface. But I don't understand how this would work together with the With command.
All the other variables /theta2, xe, ye, xu
follow from the x, y and xo values.