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The Nature of Colors

  1. Jan 25, 2010 #1
    I have been reading Eugene Hecht's Optics and the quoted text at the end of this post intrigues and confuses me. From his explanation, it seems to be that as the concentration of a dye increases, the reflected light switches to being complementary to the color of the dye.

    1.) Do painters have to worry about the concentration of a dye? If the concentration is too high then the blue sky would reflect red-green instead of blue. This might seem problematic to other applications such as printing color images and food coloring. In most cases it seems to be the solvent is intended to dry out, and the dye will become concentrated in the end.

    The quoted text is as follows from Hecht Optics 4th ed.
    "A bottle of ordinary blue ink looks blue in either reflected
    or transmitted light. But if the ink is painted on a glass slide
    and the solvent evaporates, something rather interesting
    happens. The concentrated pigment absorbs so effectively that it
    preferentially reflects at the resonant frequency, and we are
    back to the idea that a strong absorber (larger nf) is a strong
    reflector. Thus, concentrated blue-green ink reflects red,
    whereas red-blue ink reflects green. Try it with a felt marker
    (overhead projector pens are best), but you must use reflected
    light, being careful not to inundate the sample with unwanted
    light from below. The most convenient way to accomplish that
    is to put colored ink onto a black surface that isn't very
    absorbant. For example, smear red ink over a black area on a
    glossy printed page (or better yet, on a black piece of plastic)
    and it will glow green in reflected light. Gentian violet, which
    you can buy in any drugstore, works beautifully. Put some on
    a glass slide and let it dry in a thick coat. Examine both the
    reflected and transmitted light—they will be complementary."

    Thanks in advance for any contributions to this discussion.
    Last edited: Jan 25, 2010
  2. jcsd
  3. Jan 25, 2010 #2
    Painters have to consider the light they work in and the lighting for display.
    One wishes to have a studio with north windows for solar spectrum indirect.
    A painter has a closed-loop system, though- he sees what it looks like and adjusts to suit, so I'm not sure these saturation effects present any problem.

    Iodine is another thing a thin film of will make green reflections like gentian violet, but I thought that was actually iridescence due to diffraction. Is it quite certain whether it's reflection or diffraction with all the things mentioned?
  4. Feb 17, 2010 #3
    Thanks for your response.

    Here a related puzzle related to the physical nature of colors that I am confused about. Does anyone have any insights on why solid gold is yellow and colloidal gold is red? What is the physical basis of the red shift in colloidal gold? Here is a link to a picture of colloidal gold.


    Thanks in advance.

    Last edited: Feb 17, 2010
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