ICSE Class 10 Answered

Optical density is a measure of the extent to which a substance transmits light or other e-m radiations.

The optical density of a material relates to the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. The more optically dense that a material is, the slower that a wave will move through the material.