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?