Rayleigh scattering :

• The index variation causes a Rayleigh type of scattering of light. 
• Rayleigh scattering in glass in the  same phenomenon that scatters light from sun in the atmosphere, giving rise to blue sky.
• The expression for Rayleigh scattering loss is given by

αscat =(8π3/3λ2)(n2-1)2kBTfβT

n = refractive index

kB = boltzman constant

βT = isothermal compressibility

Tf = fictive temperature

λ = operative wavelength