Quote Originally Posted by bvy View Post
I see. The lensmaker's equation with radius of curvature of zero (?) for the flat side and, and r for the hemispherical side.
The radius of a flat surface is infinite, not zero (so the reciprocal of the radius is zero). You may be better off using Gullstrand's equation for a thick lens rather than the lensmaker's equation, which is only intended for thin lenses.

Best,
Helen