In which context is diffraction most commonly observed?

Diffraction is most commonly observed when waves pass through small openings or around obstacles. This phenomenon occurs because wavefronts bend and spread out after encountering an obstacle or an opening that is comparable in size to the wavelength of the wave.

When waves encounter a barrier with an opening, the smaller the opening relative to the wavelength, the more pronounced the diffraction effect. As a result, the waves can spread into the regions behind the obstacle or around the edges, creating patterns of constructive and destructive interference. This characteristic behavior is key in many applications, including the design of various types of antennas and in understanding sound propagation.

In contrast, the other choices depict different wave behaviors. Collisions between waves refer to interference, which involves the superposition of wavefronts but does not specifically relate to the phenomenon of spreading around an obstacle. Changing speed pertains to refraction, which describes how waves change direction when entering a different medium, while reflection involves the bouncing of waves off surfaces and does not lead to the characteristic bending and spreading observed in diffraction.