We know that absolute refractive index of a medium, speed of light in vacuum µ =speed of light in vacuum/speed in light in that medium We also know that speed of light is maximum in vacuum. Hence, speed of light in vacuum is always greater than speed of light in any other medium. So absolute refractive index of amedium will always be greater than 1 and never be less than 1 (else it will imply that speed of light is slower in vacuum than in that medium).