Question

what is meant by optical density,2.what is meant by propagation

Answer 1

A measure of the extent to which a substance transmits light or other electromagnetic radiation is known as optical density. 2. The act or process of propagating, especially the process by which a disturbance, such as the motion of electromagnetic or sound waves, is transmitted through a medium such as air or water.

Answer 2

see 
Optical Density (OD) is a way to describe or make you understand the blocking ability of an optical filter or a speacilly designed object
propogation mens it can be transmission of uv,iv magnetic rays etc