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if rating number 1 you divided
it should be 3x10^8 m s^(-1). here is the calculation: we have known that the refractive index of vacuum is 1, by n= v/c ( n stands for refractive index of medium, v stands for the speed of light in medium, c stands for speed of light constant) 1=v/3X10^8 m s^(-1) v= therefore, speed of light in vacuum is 3x10^8 m s^(-1) actually, all light ray in electromagnetic spectrum have same speed in vacuum
The refractive index of any substance is(speed of light in vacuum)/(speed of light in that substance) .The number is greater than ' 1 ' in any material medium.
This is called the index of refraction. When light crosses the boundary between mediums (media) with different indices, it is bent (refracted).It's usually defined the other way around ... the ratio of the speed of light in vacuum to its speed in the medium. Since the speed of light is greater in vacuum than in any medium, the number is always greater than 1. It's referred to as the "refractive index" of the medium.
It means that the speed of light in that medium is the same as in vacuum. This is ALMOST the case for stuff like "air". If you measure closely enough, it is only true for vacuum, all other materials have an optical density > 1.