refractive index同密度有冇關?

Refractive Index by definition 同 optical density 有關。
但optiacal density 同 density 是不同的呀!

A material with a higher refractive index is called an optically denser medium. When a light ray travels from an optically less dense medium to a denser medium(e.g. from air to glass), it bends towards the normal. On the other hand, when a light ray travels from an optically denser meium to a less dense medium(e.g. from glass to air), it bends away from the normal.