Optical glasses were once classified as light (crown) and dense (flint)
glass. The denser (mass per volume) glasses had higher indexes than the
light glasses. Except for some special formulations, expensive rare earth
glasses for example, it was difficult to get lighter lenses of the same
optical power by increasing the index. Is that still true? Are such glasses
used for eyeglasses?

Does the same relationship between density and index hold for plastic? Where
can I get such data on plastic? What is the mechanism that allows higher
index without a corresponding increase in density? In glass, it usually
depends upon the density of electrons that are excited by light.

Bill