- The optical density of a medium is not the same as its physical density.
- The more optically dense a material is, the slower light travels through the material and so the higher the index of refraction.
- The less optically dense (rare) a material is, the faster light travels through the material and so the lower the index of refraction.
- A vacuum has the least density and so the highest speed of light.
- Optical density accounts for the variation in refractive indices of different media.