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About white light

  1. Oct 14, 2006 #1
    Hello to all,

    White light is made up of all the wavelengths that are in the visible spectrum, and we can see it’s constituents via dispersion in a prism. My question is ;

    How are these different wavelengths kept together while light is travelling from it’s source to my eyes ?

    Is there a form of entanglement, or, are all the wavelengths emitted in the same proportion ?, that seems to me to be a bit too much to ask from a given source …

    Thank you for your response,

  2. jcsd
  3. Oct 14, 2006 #2
    There are several ways of emitting light, the most common form (for day to day experience) is whats called black body radiation. This means if something is hot it will emit EM radiation, but not just at a specific wavelength, instead it emits a spectrum, whose peak moves along the x axis as temperature increases, so you get red hot for example. But when this occurs you see red but thats not the only wavelenth being emitted, its just the others you cant see. When you get white hot the object has its intensity peak such that it covers most of the visible spectrumn, giving a white appearance.

    As far as i know the reason the object emits a spectrumn is due to the energised electrons bouncing around inside their orbitals and doing so with different energies due to a temperature distribution thats not equal everywhere. this means you get different photon emissions.

    If you look up something called the 'ultraviolet catastrophe' youll be able to learn a bit more about why this happens, also read up on blackbody radiation.
  4. Oct 14, 2006 #3


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    Funky, you just cleared up a couple of questions that have been bugging me for a while. Thanks.
  5. Oct 14, 2006 #4
    Thank you Funky, I read about black body radiation and it’s very much like you said.

    And I certainly second Danger’s comments…

  6. Oct 14, 2006 #5
    No worries :)
  7. Oct 15, 2006 #6


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    This may not be such a great coincidence as it seems. The reason light is emmitted in just the right proportions to appear white when it reaches your eye is because your eye is designed to detect light through the frequencies most abundantly available in your environment. The human eye sees light in the part of the spectrum in which the Sun puts out most of its radiation. The color we call "white" is just the experience the brain has when the optic nerves are regitering every frequency they can.

    Artificial light sources are not white by chance, either. Inventors and manufacturers run through as many substances and processes as they can think of to produce a light source that is white.

    This has always made me wonder; if intellegent life exists on a planet around a radio-emitting star, and one of them travels here to visit, what would Earth look like to him? Our Sun emits some radio, but not much. Daytime would probably look like night to him, and the Sun like a good full moon. He would probably be able to see through most walls, but unable to see the Moon, and a radio tower (if it were broadacsting on the right frequency) would look like a lighthouse.

    Just a fun mental exercise.
  8. Oct 15, 2006 #7


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    To start with, I would suspect that your alien would be one significantly dense individual. This is purely based upon the assumption that, while all stars emit radio freqencies, one who's spectrum is primarily in that range is probably either a neutron star or a black hole.
  9. Oct 15, 2006 #8


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    Remember, light rays coming from a very distant object are very nearly parallel to each other. Geometrically, that is the only orientation they could possibly be.

    When split with a prism, the light rays are now coming from a nearby "source" (the prism) and they are made to not be parallel to each other.
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