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A Waves Question

  1. Oct 22, 2013 #1
    You shine two flashlights on a wall. Why don’t you see an interference pattern?
  2. jcsd
  3. Oct 22, 2013 #2


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    Gold Member

    Why would you expect to see an interference pattern?
  4. Oct 22, 2013 #3


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    Staff: Mentor

    The light from a flashlight is a mix of many different wavelengths. Interference puts the dark and bright spots at different places for different wavelengths, so on average you end up with an equal mix of bright and dark everywhere. To get a visible interference pattern you need light sources that produce light of a uniform wavelength.
  5. Oct 22, 2013 #4


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    I asked why you expected to see interference to understand if you were having a problem understanding a specific type of interference. What Nugatory said is true in a general sense, and yet other factors come into play. The key mechanism in interference patterns are the 'destructive' and 'constructive' interactions between the waves as they cancel or reinforce each other as they meet.

    Uniform wavelength can assist in producing specific interference patterns, but isn't required. White light can produce interference patterns, too, given conditions to set up the waves to regularly destruct/construct. Soap bubbles and oil films produce interference patterns.

    Diffraction plays a big part in prepping light waves to allow interference patterns. Fraunhofer diffraction is more restricted requiring monochromatic sources. Fresnel diffraction is more relaxed, and can also create interference patterns. More here:

    Is there a particular kind of interference or experiment you are wondering about?
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