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Name: Emma S.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 3/11/2004


Question:
I am a high school student currently undertaking an assignment. My question is, how does the use of infra-red and ultra-violet spectroscopy (in identifying pigments) relate to the colour theory?


Replies:
A complete answer to your inquiry would require several books. However, the short answer is this:

Infrared spectroscopy is very useful for identifying the presence of particular types of functional groups because various functional groups tend to have similar vibrational frequencies independent of other groups that might be present. In addition, the infrared spectrum can distinguish organic pigments from inorganic mineral pigments. The organic pigments tend to have fairly sharp absorptions in the 300 to 3000 cm^-1(wavenumber) range, whereas inorganic pigments tend to have very broad absorptions. One caution here though -- a commercial pigment may contain both inorganic and organic components so this distinction may be confounded. Ultraviolet spectroscopy measures the electromagnetic radiation absorbed by the pigment in the wavelength range of about 400 to about 250 nanometers (sorry about the change from wave numbers to wavelengths, but that is the conventional way the absorptions are expressed in the two regions). In both organic and inorganic pigments these absorptions are characteristic, but are usually broad in both cases.

The ultraviolet spectrum absorptions are also characteristic of the chromophore (absorbing species) even in cases where the ultraviolet absorption is not the one that gives rise to the color of the pigment. Substances that absorb in the visible range of the spectrum -- 400 to 700 nanometers -- almost always have other absorptions at shorter wavelengths in the ultraviolet too.

How these spectra relate to the theory of color is much more involved, because the "theory" of color is very complicated because "color" is a visual perception, not an instrumentally measurable property of matter. A loose acoustic analogy would be to ask how does wave motion affect the sound of a symphony orchestra? There certainly is a relation, but a detailed explanation would be very involved.

Vince Calder



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