The spreading of waves as they pass through an aperture or around an obstacle is called diffraction.
Diffraction occurs because wavefronts do not travel only as perfectly straight rays. Under Huygens’ principle, each point on a wavefront acts as a source of secondary wavelets; at an opening or edge, those wavelets spread into regions that geometric optics would treat as shadow. The effect is strongest when the aperture or obstacle is comparable in size to the wavelength.
A single slit produces a central bright maximum flanked by darker and brighter bands. For a circular aperture, the central pattern is called an Airy disk, a limit that helps determine the resolving power of telescopes and microscopes. Fraunhofer diffraction describes the far-field pattern, while Fresnel diffraction applies when the source or screen is relatively close.
Diffraction is commonly mixed up with interference. Interference is the superposition of waves, whereas diffraction describes the spreading and resulting pattern associated with apertures and edges; mathematically, the two are closely related. Reflection redirects a wave from a surface, and refraction changes its direction when its speed changes between media.