As happens with water waves in a ripple tank, when light of a single wavelength passes through a narrow slit, the light is diffracted. In this case green light from a laser pointer is shining on a single vertical slit 0.04 mm wide. The image shows the horizontal diffraction pattern that is produced.

Diffraction of light by a pinhole or a narrow slit is one of the main pieces of evidence supporting the idea that electromagnetic radiation, including visible light, is a wave phenomenon.