Shape of the wave front of light emerging out of a convex lens when a point source is placed at its focus is hyperboloid

Shape of the wave front of portion of the wave front of light from a distant star intercepted by the Earth is plane

We can also describe the absolute refractive index as the ratio of the speed of light in a vacuum to the speed of light in an optical medium. Glass has a refractive index of 1.5, so the speed of light is 3 ??108?m/s?? 1.5 = 2 ??108?m/s.

violet component of white light travels slower than the red component