Here is a small exerpt I found while searching the internet.

"To decipher the phenomenon of refraction, we have to go down to the molecular level of substances. Optical density is directly associated with the index of refraction where the more optically dense a substance is, the higher the index of refraction it will have. The optical density of a substance correlates to the speed at which photons propagate through it. The wavelengths of photons (or EM radiation) are on the same scale as the size of the molecules. Because the molecular structure of every substance is unique, the way in which that structure interacts with the photons is also unique. This statement can ultimately conclude that the molecular structure of a substance determines its optical density, thereby determining its angle of refraction."

I seriously dont get this part: "Because the molecular structure of every substance is unique, the way in which that structure interacts with the photons is also unique."

Does anyone mind elaborating on that part? what kind of molecular structure of substances affect light refraciton differently from other molecular structures?? I am at a loss

Actually refraction is a function of something called the permittivity (symbol epsilon) of a substance, also known as the dielectric constant. It it independent of both frequency and amplitude, although substances tend to be opaque to different spectra of EM. Teflon is a good example, EM in the Terahertz range (optical light) is blocked, but EM in the radio / microwave range (Megahertz to Gigahertz) is refracted (and reflected). The optical index of refraction (symbol n) is proportional to the square root of the relative permittivity of a substance. Air has a relative permittivity (and optical index) of 1, equal to an epsilon of approx. 8.854 E-12 Farad per meter.

Hope this helps