original question: Consider light passing from one medium with index of refraction n1 into another medium with index of refraction n2. User Fermat's principle to minimize time, and derive snell's law (n1sintheta1=n2sintheta2). here is part of the solution key http://i.imgur.com/S1Pg9jC.png i understand the first line but i start getting confused when it says "Although we have v = v(y), we only have dv/dy≠ 0 when y =0" what?? i'm confused. can someone please translate that sentence for me please. the 'v' is just c, it's a constant right? and if it's a constant, and doesn't change with respect to anything WITHIN one medium, dv/dq should equal zero all the time, so what is the text referring to when it says that?