However, the new work multiplies the bit storage density of optical media ... we didn’t see any next-gen optical disc ...

The same mass takes up a bigger volume - this means the gas is less dense. Density also depends on the material. A piece of iron with the same dimensions as a piece of aluminium will be heavier ...

On this view it is clear that at ordinary temperatures the density of a solid, and therefore also its refractive index if it be of transparent material, would vary arbitrarily from point to point ...