Find out, from Table the medium having the highest optical density. Also, find the medium with the lowest optical density.
As we know optical density is the tendency to hold(absorb) the light. So
more refractive index = more absorbing power = more optical density.
It can be observed from the Table that diamond and air respectively have the highest and lowest refractive index. Therefore, diamond has the highest optical density, and air has the lowest optical density.