What best describes diffraction?

Diffraction refers specifically to the phenomenon where waves encounter obstacles or openings and bend around them. This bending occurs because of the wave nature of light and sound; as waves propagate, they can spread out in new directions when they pass through a slit or around an edge. The degree of diffraction depends on the wavelength of the waves relative to the size of the obstacle or opening — longer wavelengths diffract more than shorter wavelengths.

In the context of this question, the other descriptions do not accurately capture the essence of diffraction. For example, the change in wave speed as waves move into a different medium describes refraction, not diffraction. The reflection of waves at a boundary is a distinct phenomenon where waves bounce back after hitting a surface, and the doubling of wave frequency relates to concepts like harmonic motion or resonance, which also do not pertain to diffraction. Thus, identifying diffraction correctly as the bending of waves around obstacles is critical for understanding wave behavior in various contexts.