[SOLVED] Basic optics (GCSEs in 3 days, please help) 1. The problem statement, all variables and given/known data Here is a diagram of light passing through the curved edge of a planoconvex lens. When a light ray strikes the planar edge, it is not refracted. Why? 2. The attempt at a solution The website the diagram came from: http://www-optics.unine.ch/education/optics_tutorials/plano_convex_lens_aberration.html Light is refracted when it strikes the curved edge of the lens. I know that when light it is not refracted if it passes at 90 degrees to the border between the media. In the diagram, this isn't the case. I've studied refraction with semicircular glass plates, but light is refracted when it passes out of the straight edge. Any hints would be really useful.