What determines the refractive index of a material?

[Ravian]

1- what determines the refractive index of a material? i understand that it is ratio between velocities in vacuum and medium but i want to know about complex refractive index which also accompanies extinction co-efficient. my explanation is that bent and slowing down of light depends on crystal strucutre and arrangements of atoms and electrons in their respective orbitals. but there materials with different structures but almost similar refractive index (e.g. Al and Ag; Crown Glass and Salt) so there must be something else that determines the refractive index.

2- if refractive indices are same then are absorpton co-efficients bound to differ

3- refractive index of gold is less than one. what does it mean?

 

[Andy Resnick]

We are currently unable to predict the refractive index of a real material using microscopic theories. There are a few simple models (e.g. Drude) and a few not-so-simple (e.g. density functional theory). However, these models cannot make predictions of anything but very simple materials- crystalline solids, rarified gases, low temperatures.

The real and imaginary parts of the refractive index are related by the Kramers-Kronig relation, so if you measure one you can compute the other.

Many absorptive materials have spectral windows where the refractive index is less than 1- this often occurs in the x-ray region, where the material is strongly absorbing and dispersive effects becomes very pronounced.

 

[DrDu]

I just wanted to add that the refractive index is the square root of the dielectric function of the medium. I. e., the optical properties of a material are known once one knows the dependence of the polarization on the electric field.