Question

Which index of refraction represents the least optically dense material?

A) 2.65
B) 1.77
C) 2.42
D) 1.58

Answer 1

Higher optical density means light moves more slowly through the medium. The speed of light in a medium is given by
c' = c/n
Where n is the index of refraction. So the least optically dense material is the one with the lower index of refraction,
n=1.58