Hi, can someone please help me with the following? Describe the paths of light rays in a 2-dimensional media where the speed of light v is given by: (a) v = 1/y; (b) v = $\displaystyle \sqrt y$; (c) v =$\displaystyle \frac{1}{\sqrt y}$

Can somebody please show me how to do this type of question? I have not learned this stuff before and don't know how to start!