Dunno if the guy I was dealing with was some kinda wannbe physicist trying to test me (or just test my patents for that matter)

“So lenses wise we can do a number of different materials, 1.5, 1.6, 1.67, 1.74 at the moment your old glasses are…” “1.6 what?” “sorry that’s the refractive index. the higher the number the slower light travels threw the lens, thus the lens can be made thinner” “higher the number is slower that doesn’t make sense” “well is the speed of light in a vacuum divided by the speed of light in the lens it's sort of an simpler way of representing it coz other wise your dealing with some pretty big numbers” “do you know the speed of light?” (He’s testing me) “Well for optical calculations its normally 3x10^8 m/s (he looks at me like he’s going to correct me) well that’s a slight approximation” “yes it is…. What is it really?” “well it 299,7 something. its just a little bit under 300million meters per second” (he looks at me pleased coz ive jumped threw his hoop) “671 million Mph” (he says smugly) “well that’s an approximation too  (he looks annoyed) So anyway, it’s the speed of light in a vacuum divided by the speed of light in the medium gives you the refractive index, water is about 1.33 diamond is 2.42 but as far as we are concerned here is relates to how thin we can make lenses” “HANG ON! What do you mean speed of light in a medium? The speed of light is constant” “the speed of light in a vacuum is “a” constant yes, E = mc² c is the speed of light” “so the speed of light is constant ITS ALWAYS THE SAME” “no its constant in a vacuum but denser mediums slow it down” “the speed of light is always the same! Its constant” “it can be slowed down” “no it cant” “umm yes it can” “NO it cant” “well it seems we have reached an impasse”