GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY
Diffraction of a Helium-Neon laser beam on a small round aperture causing a classic case of Fresnel diffraction on an aperture (diaphragm, pinhole). A round aperture is placed in the path of a Gaussian beam emitted by a Helium-Neon laser. The resulting diffraction pattern is observed on a screen some distance away. Note that such truncated Gaussian beam develops characteristic intensity rings due to diffraction. The apparent laser speckle results from scattering of the monochromatic beam by the screen that is rough on a microscopic (wavelength) scale.
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