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The Spectrum and Complementary Colors


  • Standard 35 mm slide projector containing a cardboard or metal “slide” with a 1/8” center slit. This is used as the light source.
  • Spectroscopic grating
  • Reflectorized projector screen or white cardboard
  • Plastic display cases measuring 2” square and 3” tall. These provide flat windows.
  • Colored solutions. Darker or highly concentrated solutions are more effective.



A beam of light from a projector is reflected off a diffraction grating. This disperses, or spreads out, the light by color. Since white light is a mixture of all the spectral colors, one sees a rainbow-like spectral display of violet-blue-green-yellow-orange-red-deep red, each characterized by a specific wavelength.

Visible Spectrum

A white paper appears white because it reflects all the colors of light striking it. A clear liquid like water transmits all the colors of light that enters it. In either case, the eye/brain see this as white light.

A colored solution absorbs part of the spectrum and transmits the rest. The eye detects the remaining colors and the brain interprets this as a color. The color we perceive is the complementary color of that absorbed. For example, a purple colored solution (e.g., containing permanganate ion) absorbs mainly the green and some of the yellow light and allows the violet-blue and the orange-red portions to go through.

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A combination of solutions can greatly reduce the light (even produce blackness). A piece of colored paper held against the spectral display will appear black if it absorbs that color of light.

Some objects/molecular entities absorb light of a given color, i.e., belonging to a particular region of the spectrum and emit light of a color characteristic to another region of the spectrum. The phenomenon is called fluorescence. Tonic water, for example, glows blue because it contains fluorescent quinine sulfate. Another example of a fluorescent solution is a fish medicine based on the dye eosin.




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Other Demos: Chemiluminescence of Luminol, Photoresists, High Energy Emitters for Organic Light-emitting Diodes, The Spectrum and Complementary Colors, "Titrations" with Dry Ice, The Red-White-Blue Clock

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