In the famous relativity equation E mc2 the speed of light c serves as a constant of proportionality linking the formerly disparate concepts of mass m and energy E. Water n 133. Even though light slows down in matter it still travels at an amazing speed even through a dense material such as lead still travels at an amazing speed.
Your reading introduced you to the concept of index of refraction n defined by n c v where c 299792458 x 10 8 ms.
Where c - the speed of light in vacuum v - the phase velocity of light in the medium n - refractive index. V c n 30 x 10 8 m s 131 v 2290076336 m s d. Light traveling in a uniform substance or medium propagates in a straight line at a relatively constant speed unless it is refracted reflected diffracted or perturbed in some other manner. Diamond n 242.