Light travels at a velocity incredibly difficult for us to comprehend other then as just a very, very large number. To be exact, light travels at a constant velocity of 299,792,458 m/s within a vacuum. This is not the same for light traveling through other conditions. When light does travel through things other then a vacuum, there exists something called the refractive index. This index represents the amount that light is slowed. It represents this as the ration between the speed of light in vacuum and the speed at which the light travels within the medium, which is always greater then one since light can never travel faster then when it’s in a vacuum. What’s interesting is that this equation is also defined a different way. Instead of being the ration between two velocities it is defined as sin(i)/sin(r), where i is equal to the angle of incidence and r is the angle of refraction. Since this ration is always greater then 1, then it means that everything light passes through bends it in some way. This diagram from Encyclopedia Britannica represents a visual of the how light travels through a medium.