Diffraction of a Helium-Neon laser beam, a classic case of Fresnel diffraction on a slit. A narrow slit is placed in the path of a Helium-Neon laser beam and the resulting diffraction pattern is observed on a screen some distance away. Characteristic laser speckle is apparent. The speckle results from scattering of the monochromatic beam by the screen that is rough on a microscopic (wavelength) scale.