What is the bending of waves around obstacles or through openings called?

The bending or spreading of waves around obstacles or through openings is called diffraction.

Diffraction happens because waves do not always travel as perfectly straight rays. When part of a wavefront encounters an obstacle or passes through an aperture, the wave spreads into the region beyond it. The effect is especially noticeable when the obstacle or opening is comparable in size to the wavelength. Water waves curling around a breakwater and sound heard around a doorway are everyday examples.

Light also diffracts, although its very short wavelength makes the effect less obvious in ordinary situations. A narrow slit can produce alternating bright and dark bands because the wavelets from different parts of the opening interfere with one another. The central bright region becomes wider as the slit becomes narrower.

Diffraction is often confused with refraction. Refraction is the change in direction caused by entering a different medium, while diffraction is wave spreading caused by an obstruction or aperture. Diffraction can affect sound, water, light, radio waves, and even matter waves.