Relief = difference in indices
The degree of relief is proportional to the difference, in absolute value, between the refractive indices of the mediums under consideration.
In the figure, one same mineral (with a refractive index
of 1.54) is shown immersed in two oils with different indices (n=1.43 and
n=1.63). The mineral has the same relief in the two mediums with different
indices, one higher and the other lower, as in both cases the absolute difference
between the indices is similar (1.54-1.43 = 0.11 and 1.54-1.63 = 0.09).