1.  As we come down to the surface of earth from a distant star, how does the refractive index vary?

Dear Student,

Standard value of refractive index is always a ratio of speed of light in vaccum to that of medium under consideration. So, refractive index of vaccum is 1, as soon it enters Earth's atmosphere, the speed of light changes, so refractive index increases. If it enters even denser mediums, say water, refractive index increases further.