The speed of light in substance X is 1.25 x 108 m/s and that in air is 3 x 108 m/s. The refractive index of this substance will be:
speed of light in substance X is 1.25 x 108 m/s
speed of light in substance air is 3 x 108 m/s
The refractive index of this substance will be speed of light in substance air/ speed of light in substance X