What phenomenon occurs when a wave exits a small opening?

When a wave exits a small opening, it exhibits a phenomenon known as diffraction. This occurs because the wave spreads out as it passes through the opening, which is typically comparable in size to the wavelength of the wave itself. The extent of the spreading depends on the relationship between the size of the opening and the wavelength; the smaller the opening relative to the wavelength, the more pronounced the diffraction effect.

In practical terms, diffraction is most easily observed with sound waves, light waves, or water waves as they encounter openings or obstacles. For example, when sound travels through a narrow doorway, it can be heard clearly even if one is outside the room, demonstrating how the waves bend and fill the space beyond the opening.

The other phenomena mentioned do not apply in this case. Reflection involves waves bouncing back when they hit a barrier, refraction is the bending of waves as they move from one medium into another, and dispersion refers to the separation of waves into different frequencies as they pass through a medium, such as light splitting into a spectrum when it passes through a prism. Thus, the phenomenon of diffraction is specifically characterized by the spreading of waves upon exiting an opening, making it the correct answer.