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When a beam of sunlight enters a dark room, you will see a bright shaft of white highlighted by sparkles. The shaft of light is formed when some of the light is scattered into your eye by molecules of air. The sparkles are caused when some of the light is scattered into your eye by dust particles floating in the air.
Light scattered by molecules is called Rayleigh scattering. Light scattered by particles is called Mie scattering. See UIUC's web site on Atmospheric Optics for Mie Scattering images. Both these effects are named after the scientists who first discovered them.