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.
Procedure
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.
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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Remarks
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.
Remarks