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Why does light disperse?

  1. Jan 27, 2005 #1
    Why does light dispurse? is it because of interference? for example, when you shine a flashlight at wall that is farther away, why is the appearance duller?
  2. jcsd
  3. Jan 27, 2005 #2
    When light comes out of the flashlight, it can't come out perfectly straight, so you'll always have some dispersion as it propagates in space, that is the first reason.

    If light hits a wall, the minute deformities on its surface will cause the light to reflect in different directions, such that it becomes duller. This is scattered reflection, basically diffraction.

    If the wall is polished so smooth that deformities are much smaller than the light's wavelength, you have specular reflection - the case with mirrors, which are polished metal.
  4. Jan 27, 2005 #3


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    For things like flashlights, there are non-quantum factors that play a significant role. Flashlights with a lightbulb have a filiament which radiates, more or less, in a spherical fashion, and then a mirror to redirect the light to the front. Because the light bulb is not a point source, it is impossible to redirect all of the light that hits the mirror in the same direction. Moreover, there is light that goes directly from the bulb through the lens of the flashlight which will always have a fairily wide cone.

    Since this was posted in the QM forum, I will note that there are also quantum mechanics related limitations on how precise light can be, but they're AFAIK usually not noticable using flashlights. Moreover I don't know nearly enough about QM to make any informed comments about it.

    Regarding distance and light dullness:
    The flashlight produces a cone of light. If you assume that the amount of light is constant for a cut accross this cone (usually a reasonable assumption) then it can be inferred that the light per area (brightness) is less as the sections of the cone get larger i.e. further from the flashlight. If the cone is not spread out - for example, if mirrors are used to focus the light - it is quite possible to have relatively bright light at fairily long distances. Similarly, laser lights are bright for long distances because they have very narrow cones.
    Last edited: Jan 27, 2005
  5. Jan 27, 2005 #4
    thank you. I always thought that it was the photons interfering with eachother that made light dispurse.
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