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

Our perception of color is a physiological phenomenon based on red, green, and blue photoreceptors in the retinas of our eyes, and our brain has evolved to interpret combinations of intensities of these primary colors of light as different hues.   Red, green, and blue photopigments in the photoreceptors each absorb a range of wavelengths, and the absorbance spectrum below depicts their relative wavelength sensitivities. (In the sections below, we will consider how or why a pigment absorbs certain wavelengths of light. We will also explore absorption spectra in more detail.)

Figure 1: Normalized absorption curves for the S, M, and L cones. Data plotted from Stockman, A., MacLeod, D. I., & Johnson, N. E. (1993).

Consider light with λ = 575 nm will be absorbed by both the red and green photoreceptors.  The relative intensities absorbed by the red and green photoreceptors is interpreted by our brain as the color yellow.  If all wavelengths are present in light, our eyes perceive white light. In the absence of any wavelengths, our eyes perceive black.  

Analogous to the photoreceptor cones in our eyes, in the "RGB" color space, each color that our eyes perceive can be described by a red channel, a green channel, and a blue channel, each with a range of 0 to 1.   Use the interactive tool below to explore the colors associated with particular values of wavelength and with the RGB color space.   Use the λ slider to choose a color of interest. Then use the r, g, and b sliders to try to match the color of light!  

You can use the "WavelengthToColor" function to test your color matching. Enter your wavelength (in nm) and execute the two commands.

(a) Are there any colors that can be formulated with RGB but  not with a single wavelength of light? Give an example.

When white light interacts with matter, some wavelengths can be absorbed and therefore removed from the reflected or transmitted light.  (We will explore why only certain wavelengths are absorbed in the next section).   Our eyes therefore perceive the complementary color corresponding to the wavelengths that are not absorbed.  Consider the following interactive tool. The disc on the left corresponds to the color or colors that may be absorbed by a compound (or multiple compounds), such as pigments and/or dyes.  The disc on the right corresponds to the complementary color that would be observed.

Note that the complementary color for red (RGB = (1,0,0)) is cyan. (Not green as traditional RYB model teaches!)   The complementary color for green (RGB = (0,1,0)) is magenta. The complementary color for blue (RGB = (0,0,1)) is yellow.  This leads to the "CMY" primary colors for a subtractive color space, such as mixing pigments!   

(b) Assume you had three tubes of paint corresponding to cyan, magenta, and yellow.  Use the interactive tool below to determine the amounts of each hue would be needed to reproduce red, green, and blue.

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.