It is well known from experience that a light beam falling on a boundary between two different optical media splits into two light beams (reflected and refracted). When the light beam passes through a boundary between two media, its change in angle can be explained by Huygens principle. When the light wavefront hits the boundary at an angle other than 0, one part of the wave makes contact first, before other sections of the wave. That part slows down first (assuming light entered a slower environment). Since section A of the wave slowed down before the other parts, in the same period of time it traveled a shorter distance. This means that the wave front bends at part A. After all parts of the wavefront have entered the slower region, the beam has completely bent into a slightly different direction.