Question

How does the average speed of light in glass compare with its speed in a vacuum?

Answer 1

the speed of light in air is 3*10^8m/s and the refractive index of glass is 1.5
 so the speed of light in glass is calculated by the formula
refractive index =c/v
 3*10^8/1.5= speed of light in glass
 speed of light in glass=2*10^8m/s