Question

why refractive index can't be less than1 ??

Answer 1

refractive index is the ratio of speed of light in vacuum to that of the denser medium because any medium is denser than the vacuum even air so 

speed of light is greater in vaccum and small comparatively in the denser medium. 

so we can take a general idea that refractive index can't less than 1

Answer 2

refractive index can be less than 1, like in X- rays regime