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Spectrum of light

  1. May 3, 2005 #1


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    ok im asking this for a friend, but anyway:

    light consists of 7 basic colors (ROYGBIV)

    the question is quite simply why 7? why isnt brown in the spectrum? (note i tried to explain that brown is a combo of the other wavelengths but apparently the question is more like *why* is it only 7? and why isnt brown in the list? and why isnt it like 95?)
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  3. May 3, 2005 #2


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    I think we have had this discussion. 7 colors came out of Newtons bent to numerology. There is no real reason for it. I am unable to locate an Indigo band in the rainbow, I tend towards 6 bands rather then seven.

    Look at a rainbow, do you see brown? That is why it is not in the list of primary colors, as they are essentially the colors of the rainbow. You must keep in mind that what we call color is simply our senses reaction to certian levels of EM energy or combinations of different energies.

    The key answer to why there are 7 (or 6) is in the rainbow.
  4. May 4, 2005 #3
    is brown light nearly wight light, say a mix between blue, green and indigo light, generally, you don't get brown light sources, only brown pigments, and those are a mix of a primary colour and a secondary colours
    wikipedia- "Brown is a color produced by mixing small intensities of red and green, orange and blue, or yellow and purple pigment. Brown exists as a color perception only in the presence of a brighter color contrast."
  5. May 4, 2005 #4
    According to MSpaint, brown is made up of 50% red light and 25% green.

    Goes to show paint is useful for some things
  6. May 4, 2005 #5


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    well like because theres 7, does that mean that light is broken up into 7 pieces ? or x pieces? (who cares about 7). like is it non continuous in some way? thats the real question
  7. May 4, 2005 #6


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    The visible spectrum is very nearly continuous, again observe a rainbow. Discontinuities are at the quantum level, far below the sensitivity of your eye. In other words, as far as your eye is concerned he spectrum is continuous.

    You computer monitor only knows about 3 colors, Red, Green and blue. From those 3 you can recreate the visible spectrum.
  8. May 5, 2005 #7


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    ok *but* im asking why did people decide that there are 7 colors there? like granted its continuous, so why didnt we choose breen and yeller instead of our normal colors? is that because we can more easliy tell them apart?

    sorry if this is a bit repetetive but i had an exhaustive talk that pretty much followed this when i talked to him about it. in the end i didnt satisfy him so i asked here :)
  9. May 5, 2005 #8


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    That question was answered in my very first post.
  10. May 5, 2005 #9
    This is not a physics problem, instead, this is related to the human eyes...

    Human eyes have 3 different color sensor(red, green and blue), every wavelength in the visible light will turn on 1 or more color sensor...
    The reason you see 7 color in rainbow is because there are 7 different combination of the on/off pattern.... can you count them all??
  11. May 5, 2005 #10

    Doc Al

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    Very interesting!

    In addition to his numerological bent, as noted by Integral, Newton also divided the spectrum into 7 parts based on ratios of the 7 musical tones in the major scale.
  12. May 5, 2005 #11
    Arbitrary, 6,000

    The division of the visible spectrum into seven colors is purely arbitrary. I have read that a person can distinguish about 6,000 different colors. In fact, color identification systems, like the Munsel system, distinguish thousands of colors.

    My guess about Roy G. Biv is that they started with the additive primary colors Red-Yellow-Blue, added the secondary colors Orange-Green-Violet and added Indigo to make BV into a word that could be a name. Did you ever know anyone named "Biv?"

    The McGraw-Hill Encyclopedia of Science and Technology, available at many libraries, has an excellent article on color. I'm sure Encyclopedia Britannica does, too.
  13. May 5, 2005 #12
    P.S. Brown

    Brown is dark yellow.

    As someone indicated above, you can create brown by mixing red and green in the subtractive color system.

    Play around with this in a program like MSPaint, or Word, etc. that lets you specify intensities for R G and B.

    Windows lets you do this.
    Right click on the Desktop;
    select Properties;
    click on the Appearance tab;
    click on Advanced;
    click on Color.
    That is: Desktop -> Properties -> Appearance -> Advanced -> Color.

    Set Red to 0 and play around with Red and Green. You'll find low values of green and blue give brown and high values for both give yellow.
    Last edited: May 5, 2005
  14. May 7, 2005 #13


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    thanks. i hope this will pacify him :)
  15. May 7, 2005 #14
    Actually, I find the whole idea that we can even sense light at all fascinating. Nothing more than our version of optical echolocation, a way of differentiating matter. Give us two eyes, and we can also get a rough approximation of distance.

    But our "visible spectrum"? It could have been anything, really. If we could sense infra-red or ultra-violet with our naked eyes, our brain would assign them gradations of "color", to differentiate them from other wavelengths. Likewise, if your optic nerves could sense wavelengths just below infrared (microwave), or just above ultra-violet (X-ray) no doubt we would see those as separate "colors" as well.

    Can't even fathom - like trying to describe color to one who was born blind.
  16. May 7, 2005 #15
    Why red to violet?

    That we can see electromagnetic radiation that ranges from red to violet is not a chance occurrence. The atmosphere is especially transparent to those wavelengths.

    Some animals, honeybees for example, also use ultraviolet. Some flowers look markedly different to bees. Others use IR--like rattlesnakes who use IR sensors to fine tune their movement during a strike.

    Presumably, humans might have evolved to sense UV and IR. I don't know why we didn't. Evidently, as often occurs in evolution, red to violet was good enough.

  17. May 7, 2005 #16
    Screw that. I want more. Same with my ears. Sick of envying dogs.
  18. May 7, 2005 #17
    Food coloring

    My "P.S. Brown" posting above is a little screwy. Red and green make yellow. To play around with a computer program, set blue to zero and change values for red and green.

    Another way to observe brown as being dark yellow is to get a bottle of yellow food coloring. You'll notice that it looks brown in the bottle. That is because there is so much dye in the water that it absorbs just about all the light.

    Put some yellow food coloring on a white plate. It will look brown, as it did in the bottle. Add water to it, one drop at a time. The brown will lighten to yellow.

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