'm currently reading chapter 27 of the Feynman lectures, where he derives the thin lens equation for a single refracting surface. He arrives at the equation h^2/2s + nh^2/s' = (n-1)h^2/2r Where s is the object distance, s' is the image distance, n is the index of refraction of the second medium (the first is one), r is the radius of curvature, and h is the altitude of a triangle Where does the (n-1) come from? Wouldn't it simply be n? This is kind of vague, so please look at the second page of http://student.fizika.org/~jsisko/K...on Physics/Vol 1 Ch 27 - Geometric Optics.pdf I would be so grateful. Thanks.