How Refraction Makes This Beaker Disappear Refraction is the change in direction of a wave, typically light, as it passes from one medium to another with a different optical density, due to a change in wave speed. When light travels from a less optically dense medium (like air) into a more optically dense one (like glass or water), it slows down and bends toward the normal (a line perpendicular to the boundary surface). This phenomenon is governed by Snell’s Law, expressed as n₁sinθ₁ = n₂sinθ₂,