Re: Why does polycarbonate have a higher refractive index than crown glass?

Refractive index is dependent on other factors also.  

The simple Lorentz-lorenz relationship is often used to describe the R.I.
of materials.  The parameters in this relationship are the molar
refractivity and pressure variation with temperature. The molar
refractivity can in turn be expressed as a function of the polarizability
(in this case proportional to polarizability) of the material.
Polycarbonate used in high-index lenses is prepared by crosslinking allyl
(-CH2-CH=CH2-) functional groups which have high polarizability. My first
guess is that the high R.I of the polymer arises from the intrinsically
high polarizability that is present. 
The very reason why polycarbonate is used in high-index lenses is its light
weight compared to crown glass.