What is optical density?  

Optical density of a medium is the ability of that medium to refract light. Medium in which speed of light is more as compared to the other mediums is called optically rarer medium and the one in which speed of light is less is optically denser.

The refractive index of optically denser medium is more as compared to R.I of optically rarer medium.

Medium having greater value of refractive index is called optically denser medium, this means light will travel at slower speed in optically denser medium compared to in an optically rarer medium.

The optical density of a medium is not the same as its physical density. The physical density of a material refers to the mass/volume ratio. 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.