By the end of this lesson, students will be able to:

In this lesson, we begin to answer the question: "Why are some compounds colored?".   In this section, we use a more 'macroscopic' approach to answer the question.  In another lesson, we will look at a more 'microscopic' approach.

In the lesson on "Color and Absorption" we found that if we are observing a color, then its complementary color must have been absorbed.  Therefore, if a compound is colored, then it must be absorbing certain wavelengths of visible light and reflecting/transmitting others!

Consider a theoretical pigment or dye that absorbs in the visible spectrum.  Assume we illuminate a sample containing the compound with light. Some of the light at particular wavelengths will be absorbed.  If we know the intensity, or brightness, of the incoming light at each wavelength, we can measure the intensity of transmitted light at each wavelength to measure how much was absorbed.  A plot of the amount of light absorbed at at each wavelength in a given range is known as an absorption spectrum.  UV/Vis absorption spectroscopy is a powerful tool for studying the electronic properties of molecules.    

Figure 1 depicts this absorption process for a particular dye, 3,3'-diethylthiacyanine iodide, and the corresponding absorption spectrum.  The blue wavelengths from incoming light from the left are absorbed by the dye molecules in the sample, and the transmitted wavelengths, centered about yellow, are what we observe.  The plot at the right is the corresponding absorption spectrum, again showing that blue wavelengths are absorbed.

Figure 1: Cartoon depicting absorption process for dye, 3,3'-diethylthiacyanine iodide and the corresponding absorption spectrum.

The y-axis in the absorption spectrum corresponds to absorbance (A), which is related to the ratio of the incoming intensity, I₀, at each wavelength to the transmitted intensity, I:

A =$\log \frac{I{}_0}{I}$

Note that several factors may affect how much light is absorbed at each wavelength. The concentration, C (molarity, mol/L), of the dye is a measure of how many absorbing particles are present. The higher the concentration, the more dye molecules are present, the more photons will be absorbed, the higher the absorbance.  The path length, l (cm), is related to the distance light must travel through the sample. The longer the distance, the more absorbing particles will be present, the higher the absorbance. For UV/Vis instruments that utilize a common cuvette (as depicted in Figure 1),  the pathlength is l = 1 cm. Finally, the molar absorptivity, e(l) (units = L$ mol^-1$ cm^-1), is related to how strongly the compound absorbs photons at the given wavelength.   For dilute concentrations, these variables are related by Beer's law:

A = e$l$C.

In Beer's law, the only variable that depends on wavelength is the molar absorptivity, e.

Notice that the absorption peak in Figure 1 can be somewhat broad, absorbing over a range of wavelengths.  The line shape can be due to the pigment or dye absorbing at multiple wavelengths or interactions of pigment/dye molecules with the solvent or with themselves.  The overall observed color then is related to the line width.

When discussing the color value of a sample, it is common to express three parameters: the hue (color), the chroma (or the richness or purity of a color), and the value (its desaturated value on a gray scale.)  While these parameters are not explicitly represented in an absorption plot, there are some connections that we can make.  The observed hue is related to the peak wavelength being absorbed (namely related to the complementary color of the one being absorbed, as we have discussed at length).  The chroma is related to the line shape.  The sharper the peak, the higher the chroma.  

Consider a theoretical pigment or dye that absorbs in the visible spectrum. Use the commands below to relate a UV/Vis spectrum to the observed color of pigment or dye. (Note that this activity represents a simplified or approximate connection between a UV/Vis spectrum and the observed color.)

Step 1: Use the interactive tool to create an absorption spectrum of a theoretical pigment or dye.  The hue is determined by the wavelength. The chroma is related to the peak width, which can be controlled by the parameter σ.  The intensity is related to the molar absorptivity, e.

Step 2: Enter the values for λ, ε, and σ from the plot above and execute the commands to see the corresponding absorbed color (left) and observed color (right):

Notice that as the line width (sigma) gets more broad, the observed color is less rich, corresponding to less chroma!

1. Christie, R. The Physical and Chemical Basis of Colour. In Colour Chemistry. 2nd Ed. Royal Chemical Society: Cambridge. 2001. pp. 12-21.
2. Stockman, A., MacLeod, D. I., & Johnson, N. E. (1993). Spectral sensitivities of the human cones. Journal of the Optical Society of America, A, Optics, Image & Science, 10(12), 2491–2521.