I was reading on thin lenses, in which the lens makers' equation was proved. The proof first gave the lens (a biconvex lens) a thickness (which it would later consider negligible) so that the light would pass through one surface and out the other (so it comes into contact with 2 surfaces). The proof was based on the postulation that the image formed from contact with the 1st surface functions as the object for the 2nd surface (and vice versa). The postulation seems true for when the 1st image formed is virtual and behind the actual object, but it doesn't seem to make sense for when the 1st image formed is real and behind the surface. Can anyone clarify how the second case works?