Optical Density

Different transparent media are found to transmit radiant energy, e.g., light, with different velocities, depending upon the nature of the substance. Of two media that one is said to have the greater optical density in which the velocity of propagation is the le . Again, if a ray of light be incident upon the surface separating two media at any angle between zero and the critical angle, that medium has the greater optical density in which the ray makes the le angle with the normal at the point of incidence. Optical density and ma density often correspond, though there are several notable exceptions.