- The optical density of a medium is not the same as its physical density.
- The more optically dense a medium, the slower light travels through it.
- The less optically dense (or rare) a material is, the faster light travels through it.
- A vacuum has the least optical density and so light travels through it at a maximum speed of 299,792 kilometres per second.
- Optical density accounts for the variation in refractive indices of different media.