Light travelling through three transparent substances follows the path shown in figure.Arrange the indices of refraction in order from smallest to largest. Note that total internal reflection does occur on the bottom surface of medium 2

As the ray moves toward the normal while entering medium 2 from 1, we have n2> n1For total internal reflection at interface of 2 and 3, n2 > n3.Besides n3should also be less than n1or else ray would haveemerged in medium 3, parallel to its path in medium 1.Hence, n3 < n1 < n2 is the correct order.