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Why does light bend when it enters glass or water?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Light bends when it enters a different medium, such as glass or water, because its speed changes. This phenomenon is known as refraction.1

When a light wave crosses the boundary between two transparent substances, the change in speed causes the wave to alter its direction. The degree of this bending is determined by the refractive index of each medium, which is defined as the ratio of the speed of light in a vacuum to its speed within that specific material.123

According to Snell’s law, when light enters a medium with a higher refractive index—meaning the light travels more slowly—the ray is bent toward the normal, an imaginary line perpendicular to the surface. Conversely, if light moves into a medium with a lower refractive index, it speeds up and bends away from the normal. This behavior is a fundamental characteristic of electromagnetic radiation as it interacts with matter.4215