MadSci Network: Physics

Re: Colors; wavelengths not adding up for me

Date: Mon Sep 7 01:25:00 1998
Posted By: Werner Sieber, Research Scientist,Ciba Specialty Chemicals Corp.
Area of science: Physics
ID: 904104382.Ph

Dear Claridge,

"Red", "green", "yellow".. are descriptive terms which we commonly apply to apples, oranges, pearls, peacocks as well as rainbows and images on computer screens. Colors, however, are NOT inherent properties of objects, but sensations perceived by our brain when our eyes are exposed to electromagnetic radiation of very specific wavelength range and intensity distribution. Thus there are by definition no COLORS THAT CANNOT BE SEEN by the human eye. Of course there exists a wide realm of electromagnetic radiation which our eyes do not perceive. But for this very reason we cannot assign "colors" to these wavelength bands (SOUNDS: see below). In the lab it should also be possible to create visible light by mixing rays from outside the visible spectrum, but only using COHERENT beams, i.e. lasers. Light from the sun and from any ordinary lamps is incoherent: it consist of small "packets" with random phase and polarization. The reason why we can recognise objects by "their color" is the (relative) constancy of composition of sunlight, which we perceive as white. (Try to judge the ripeness of apples in moonlight!) Colored object surfaces, like paints and inks, SELECTIVELY remove (absorb) part of the incident white light. The REMAINING MIXTURE of radiation, which is reflected by the object, is perceived as colored. The curve which describes which wavelengths are preferentially absorbed is called the ABSORPTION SPECTRUM, and is an inherent property of the object.

The real bewilderment starts when coffee-break discussion turns to "color mixing". Red, blue and yellow (or more accurately magenta, cyan and yellow) are indeed primary colors, as far as SUBTRACTIVE MIXING is concerned. If you mix 3 INKS of these respective colors in equal proportions, you get BLACK, because each ink roughly absorbs its third of the visible spectrum, and no light is left. If you only mix cyan and yellow inks, you get green, because this mixture blocks red and blue parts of the daylight spectrum, etc. "Object colors" may thus be thought of as COLOR FILTERS. If you superpose filters, you get subtractive mixing, i.e. remove one wavelength band after the other. Color prints of photographs are structured similarly using cyan, magenta, and yellow dye layers.

ADDITIVE mixing rules apply when we deal with mixing LIGHT BEAMS of different wavelength bands, e.g. those emitted by the tiny phosphor dots on computer screens. Primary colors in this case are red, green and blue (RGB), because 3 light beams of the corresponding wavelengths ADD UP to...WHITE! (figure out yourself the PAIRWISE mixings).

SOUND is not an electromagnetic radiation. Sound waves (as opposed to light) only exist in matter (gas, liquid or solid), most commonly in air. Sound waves are much more COHERENT than light, and sum and difference frequencies are more easily observed, also because the sensitive range of our ear is much wider (3 powers of 10 in frequency) than that of the eye (a factor of 2). The frequency ratio of the "red" and the "purple" ends of the light spectrum would thus barely correspond to one octave in music. Mixtures of sound of closely spaced wavelengths are generally perceived as NOISE. WHITE NOISE is the extreme case, and the term is an analogy from the realm of visual perception.

This is all very brief, but I hope it helps you along. One thing is sure about visual perception: there is more to it than meets the eye... An excellent new book on the subject is "COLOR VISION, Perspectives from Different Disciplines", W.G.K.Backhaus, R.Kliegl and J.S.Werner, eds, Walter de Gruyter, Berlin, New York 1998.

Best regards,
Werner Sieber

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