Which phenomenon occurs when light changes speed and bends as it enters a different medium?

When light transitions between different mediums, such as air to water or air to glass, it changes speed due to the varying optical densities of the materials. This change in speed leads to a bending effect, which is known as refraction. The principle behind refraction is based on Snell's Law, where the angle of incidence and the angle of refraction are related to the indices of refraction of the two media involved.

The bending of light is what allows us to see objects clearly when they are submerged in water or why a straw looks bent when placed in a glass of water. Refraction is a key concept in understanding various optical phenomena and is critical in applications like lenses in glasses and cameras. Thus, option C is the correct answer for the phenomenon that describes this behavior of light.