diffraction

Diffraction is the spreading of a wave after it encounters an obstacle, a sharp corner, or emerges from a slit. If the slit is small, the spreading is large and is accompanied by an interference pattern with a central peak surrounded by weaker side lobes. In this context, "small" means comparable to the wavelength of the diffracting wave. The fact that light diffracts when passed through a small slit is evidence of its wave nature.