Find the medium having highest optical density. Also find the medium with lowest optical density.

(a) The medium having highest refractive index has the highest optical density. Since diamond has highest refractive index of 2.42 Thus, diamond has the highest optical density.

(b) The medium having lowest refractive index has the lowest optical density. Since air has the lowest refractive index of 1.0003 Thus, air has the lowest optical density.