• Question: Why does a laser beam look fuzzy?

    Asked by kaur to Mike, Pip, Tianfu, Tim, Tom on 27 Jun 2012.
    • Photo: Tom Lister

      Tom Lister answered on 27 Jun 2012:

      Some of the bits of the beam add together to make bright bits and some add together to make dark bits (cancelling each other out). This is changing all of the time, so it looks fuzzy.

      If you have ever sat near the sea on a stormy day you may have seen waves bouncing back off the cliffs and then either turning to nothing as it hits another incoming swell or splashing right up into the air as it hits the peak of another wave- this is what we call ‘superposition’.

    • Photo: Tim Stephens

      Tim Stephens answered on 27 Jun 2012:

      The fuzziness you see is an effect called “Speckle”. Speckle is what we call an interference effect.
      What this means, like Tom says, is that light from different parts of the beam reflects back to your eye, and some waves add together to make bright spots whilst others cancel out to make dark spots.
      You might have seen the same effect on a pond or on the sea where some waves add together to make higher peaks, whilst others cancel out. I remember being shown these in a little wave tank when I was in school.

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