The phenomenon that occurs when light waves spread out after passing through a narrow slit is known as diffraction. Diffraction is a wave property that causes a wave to bend around obstacles and spread out as it passes through an aperture. This effect is most pronounced when the width of the slit is comparable to the wavelength of the light. Diffraction plays a crucial role in various optical phenomena and can be observed in experiments such as the double-slit experiment, which illustrates fundamental principles of wave-particle duality.