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Please explain the appearance of a laser

  1. Oct 26, 2014 #1
    When you look at the spot of light made by a laser pointer why does the light have a shimmering grainy look?
  2. jcsd
  3. Oct 26, 2014 #2


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    Staff: Mentor

    That grainy appearance is caused by interference. At the points where a trough of the wave and a crest both land, they cancel out and you get a dark region; where two troughs or two crests land they reinforce and you get a bright region.

    You only see this with laser light because a laser produces coherent light, all with the same wavelength and same phase. A flashlight beam or other ordinary light source won't produce this effect because it produces a mix (called "incoherent" light) of different wavelengths and different phases, so at every point it averages out to a uniform brightness.
  4. Oct 26, 2014 #3


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    Science Advisor
    Gold Member

    The light waves constructively and destructively interfere with one another based on the surfaces they bounce off. This causes the intensity of the diffusely reflected light to be alternately enhanced and diminished, and this effect is what gives lasers their characteristic mottled look. As a side note, the reason you don't typically see the grainy pattern with regular white light is because white light usually isn't coherent (the light waves don't line up like they do in laser light), and it's made up of a wide spectrum of colors (lasers have a very narrow bandwidth--they're functionally monochromatic).
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