What phenomenon describes the spreading of light waves?

The phenomenon that describes the spreading of light waves is diffraction. This occurs when light encounters an obstacle or opening that is comparable in size to its wavelength. As a result, the light bends around the edges of the obstacle or spreads out after passing through the opening. This spreading effect can lead to various patterns of light and shadow, which are often observed in experiments involving slits and diffraction gratings.

Reflection involves the bouncing back of light waves when they hit a surface, while refraction refers to the bending of light as it passes from one medium to another, due to a change in its speed. Interference is a phenomenon that occurs when two or more waves overlap and combine, resulting in a new wave pattern. Although these concepts are related to the behavior of light, they do not specifically describe the spreading of light waves like diffraction does. Thus, diffraction is the correct answer in this context.