## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

We can write an expression for the speed of light in a medium: $v = \frac{c}{n}$ where $n$ is the index of refraction of the medium. If the refractive index is higher near the center, the light travelling near the center will travel more slowly. Note that the path length near the center is a shorter distance. If the refractive index is lower near the edge, the light travelling near the edge will travel more quickly. Note that the path length near the edge is a longer distance. The light traveling a shorter path at a slower speed near the center will reach the end at about the same time as the light traveling a longer path at a faster speed near the edge.