I am aware this question has been asked many a time but i have never found a answer which satisfied my exact requirements. I recently completed an experiment on how refractive index is affected by temperature. I found that as temperature increases, refractive index decreases. I thought this was due to a density change and the fact there are less molecules that the light ray is absorbed and remitted by. However my teacher believed that i should look further into optical density. I know this is the sluggish tendency of the molecules but i don't know how to link these two definitions to give a correct scientific background context. I was also told to look into relative permittivity. If someone could give me a push in the right direction i would be very grateful.