Three questions: 1) Anyone have a pedagogic answer to why the image formed by a thin lens is inverted (i.e. upside-down)? I realize that to get a focused image, the lens has to converge the rays one way or another, so then, eventually, the rays have to cross the optic axis, which in turn leads to the upside down image. But couldn't the image be formed before the rays cross the optic axis? Maybe not, because there it cannot be focused, but why can't it be focused there? 2) Is it possible to construct a lens (or any single transparent material) that actually forms images which are not upside down? (Note: Several lenses in series are not allowed.) 3) That the image seems to have to be inverted really puzzles me - is there some deep fundamentality in nature which predicts this inversion behaviour of an optical system like a lens? Or is it more of "that's just the way it is"?