What phenomenon occurs when a ray of light changes direction as it enters a different material?

Refraction is the phenomenon that occurs when a ray of light changes direction as it passes from one material into another with a different density. This change in speed as light travels through different media causes it to bend. For example, when light travels from air into water, it slows down and alters its angle, which is perceived as refraction. This principle is essential in optics and is the reason why lenses can focus or disperse light, leading to various applications in eyewear, cameras, and other optical devices.

In contrast, diffraction refers to the bending of light around obstacles or through openings, which is not relevant to the change in direction at an interface between two different materials. Reflection involves the bouncing back of light when it hits a surface, which does not entail a change in medium, and absorption concerns light being taken in by a material rather than passing through it. Thus, refraction is specifically linked to the change in direction due to the transition between materials, making it the correct answer.