What phenomenon describes the bending of light as it enters a dense medium?

The phenomenon that describes the bending of light as it enters a denser medium is refraction. This occurs because light travels at different speeds in different materials; it slows down as it moves from a less dense medium (like air) to a denser medium (like water or glass). When the light enters the denser medium at an angle, it changes direction due to this change in speed. The amount of bending depends on the angle of incidence and the indices of refraction of the two media involved, according to Snell's Law.

In contrast, reflection involves light bouncing off a surface rather than bending. Transmission refers to light passing through a medium without being absorbed or reflected, while diffraction is the spreading of waves when they encounter an obstacle or opening. Refraction specifically addresses the bending of light due to changes in medium density, making it the correct answer in this context.