UV, IR, and Color Theory
Name: Emma S.
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
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.
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Update: June 2012