The direction of light changes when it passes from one medium to another due to a phenomenon known as refraction. Refraction occurs because light travels at different speeds in different materials. When light encounters a boundary between two media with different optical densities, such as air and water or air and glass, its speed changes, and as a result, its direction also changes.
The change in speed of light is related to the change in the refractive index of the materials involved. The refractive index is a property of a material that describes how much the speed of light is reduced when it enters that material compared to its speed in a vacuum. Different materials have different refractive indices.
When light passes from a medium with a lower refractive index to a medium with a higher refractive index, it slows down and bends toward the normal, which is an imaginary line perpendicular to the surface of the boundary. This bending is what causes the change in direction of the light.
Conversely, when light passes from a medium with a higher refractive index to a medium with a lower refractive index, it speeds up and bends away from the normal, resulting in a change in direction.
This change in direction is a fundamental property of light and is governed by the laws of refraction, specifically Snell's law. Snell's law mathematically describes the relationship between the angles of incidence and refraction and the refractive indices of the two media.
In summary, the direction of light changes in refraction because of the change in speed as it passes from one medium to another with a different refractive index.