ior_of_material = Lightspeed_in_void / Lightspeed_in_material Lightspeed_in_void is light's maximum speed. 300 000 km/sec If i recall well. Lightspeed_in_material can't be higher than Lightspeed_in_void, so ior_of_material can't be lower than 1. I guess this could be an Elysiun question and an Elysiun answer. Dani