What phenomenon occurs when a wave spreads into the geometric shadow of an object?

The phenomenon that occurs when a wave spreads into the geometric shadow of an object is known as diffraction. This behavior is characteristic of waves, including sound waves, light waves, and water waves, allowing them to bend around obstacles and fill areas that are not in direct line with the wave source.

When waves encounter an obstacle, if the size of the obstacle is comparable to the wavelength of the wave, the waves will not simply travel in straight lines but will instead bend around the edges of the obstacle and continue into the shadowed region. This results in a spreading pattern that can be observed, demonstrating the wave nature of the phenomenon.

The other options relate to different wave interactions: refraction involves the bending of waves as they pass from one medium to another with different densities, interference refers to the effect that occurs when two or more waves overlap and combine, and reflection occurs when waves bounce off a surface. None of these processes specifically describe the spreading of waves into a shadowed area, which is uniquely defined by diffraction.