Show that the shortest distance between two points on a plane is a straight line. 6-7. Consider light passing from one medium with index of refraction 111 into another medium with index of refraction n2 (Figure 6-A). Use Fermat's principle to mini- mize time, and derive the law of refraction: n1 sin 61 = n2 sin 92. I l I | I I | I | I I l I I l l I I | I I I I I | I FIGURE -A Problem 6-7. 5-10. Find the ratio of the radius R to the height H of a right-circular cylinder of xed volume V that minimizes the surface area A