Refraction is the change of direction that a light ray experiences when passing from a transparent medium (water, glass, air) to another where the velocity of propagation of the wave is different. For example, light propagates more slowly in any medium than in a vacuum. Based on this, there are indices of refraction for each material, which are actually a comparison between the speed of light of a transparent medium and that of the vacuum. For example, the refractive index of water is 1,333. This means that in a vacuum the speed of light is 1,333 times faster than in water. Therefore when a ray of light passes from a medium like air to water, it changes its direction.