Light refracts because its speed changes when it passes from one medium to another. This change in speed causes the light wave to change direction at the boundary between the two media. When light enters a medium where it travels slower (like from air into water or glass), it bends toward the normal line, and when it enters a medium where it travels faster (like from water to air), it bends away from the normal line. This bending is due to the wave nature of light and the interaction with the electrons in the medium, which slows the effective speed of the light wave and alters its propagation angle.