refractive index of vacuum is 1 so from this law: n=c/u,we see that u of any light will be equal with c=3*10^8. When two lights(Red(r),Blue(b)) run into the water,red will run faster because refractive index for this lightwave is less than refractive index of blue. So we see that refractive index except of material,has to about lightwave too. My question is: Why in all materials,refractive index is different for each lightwave,but in vacuum refractive index is the same for all ligthwaves(so red and blue run with same speed at vacuum)?? Thanks!!!