The number that indicates how optically dense a substance is is called the refractive index.

Here's how it works:

* Refractive index is a measure of how much light bends when it passes from one medium to another.

* A higher refractive index means the light bends more, indicating a denser medium.

* For example, water has a refractive index of 1.33, while diamond has a refractive index of 2.42. This means light bends more when passing through diamond, making it appear more "sparkly".

So, the higher the refractive index, the denser the substance is optically.