Why does light bend toward the normal when it enters a denser medium?

Light bends toward the normal when entering a denser medium because the speed of light decreases in that medium.1

According to Huygens’s principle, light propagates as a series of wave fronts where each point acts as a source of new wavelets. When a light beam strikes the boundary between two media at an angle, the part of the wave front that enters the second medium first slows down, while the part still in the first medium continues at a higher speed.1

Because the wavelets in the denser medium have a smaller radius than those in the less dense medium, the overall orientation of the wave front changes. This change in speed causes the direction of the light ray to pivot toward the normal line, which is the perpendicular line at the boundary. This relationship is mathematically described by Snell’s law, which relates the angles of incidence and refraction to the refractive index of each material.1