Per questions in the Thursday Chem-15 lab, here is some additional information you will
find useful, not in the text. Some of the questions in the experiment ask you to identify
colors associated with particular wavelengths. Wavelengths of colors can be found
several places on the web, here are a few examples. Note that specific wavelengths can
have slightly different “color interpretations”. Red is usually considered as 650 nm.
It also turns out that not all colors are “real” in the sense that they have a specific
wavelength. Magenta, for example, is a mixture of red and blue (from opposite ends of
the visible spectrum) which our eye-brain perceives as a new color ... but it has no real
wavelength, just two peaks in the red and blue which our vision combines.
Part of our experiment utilized combinations of food dye solutions. Predictions of
resulting color can be made on a “color wheel”. Here is some additional detail on use of
the wheel.
What happens if we make an additive mixture of equal amounts of green and violet?
To gain a sense of the resulting hue, first draw a straight line between green and violet, as
seen in the figure above. Then place a dot on the line halfway between the two colors.
Next, draw a straight line from the middle of the color circle to the dot. The resulting line
points towards blue, with a green tendency. To predict an additive mixture with a lot of
violet and relatively little green, you would place the dot closer to violet. Notice that the
resulting hue would be closer to blue. Similarly, try a mixture of mostly orange, with just
a little green. Notice that the result is yellowish-orange. The color wheel below provides
actual colors to predict mixtures, with a slightly different arrangement around the wheel.
Note that when a particular color is absorbed (removal), the resulting transmission color
is towards the opposite side of the wheel. Thus a solution which absorbs yellow appears
blue when we look through it, so it would have high Transmittance in blue, and high
Absorption in yellow. This is a key idea in answering the experiment questions.
Beer’s law is the linear relationship between absorbance and concentration, and gives a
straight line when plotted. In our case, “concentration” was qualitative, since we don’t
know the molar concentration of dye in the grocery store food color, but we do know
how we diluted it. Thus the concentrations would be 1/10mL, 2/10mL, 3/10mL, etc. (e.g.
0.1, 0.2, 0.3 ...) which can be used to make the plot of concentration versus absorbance.
Remember that we use a calculated Absorbance value from Transmittance measurements,
because the Absorbance scale on the spectrophotometer is a highly non-linear log value
making it hard to read and therefore likely to be inaccurate
It also turns out that not all colors seen are “real” in the sense that they have a specific
wavelength. Magenta, for example, is a mixture of red and blue (from opposite ends of
the visible spectrum) which our eyes + brain perceive as a new color ... but it has no real
wavelength, just two peaks in the red and blue which our human vision combines. Our
spectrophotometer sees it as a combination of two absorptions rather than a single color.
Color in practice is a combination of actual wavelengths and human perception. The
chromaticity diagram below attempts to combine the two, showing what the eye sees in
colors and the associated wavelength (if real). In the lab we are focused more on
wavelengths more than perception, so we rely on the spectrophotometer, which separates
magenta into its component colors.
Here’s an example of “Chromaticity Diagram’, used to analyze colors for printing and
monitors according to limits of human perception. The diagram traces spectral colors
(ones with a wavelength) around the perimeter, but the straight line connecting blue and
red are non-spectral colors, without a wavelength assignment. Our brain combines red
and blue colors into “purple”, but the spectrophotometer does not.
Finally, here is some energy data, which might be useful for lecture material, illustrating
the more energetic nature of blue colors.