iation by heated solids. The constant h has the value 4.136 X 10 15 eV/s, and is referred to as the Planck constant, while f represents the frequency of the photon. We can also express the energy of a photon in terms of its wavelength and the speed of light in a vacuum. Ignoring intermediate steps and fol- lowing the expression of Indiana Jones ("Trust me"), we can rewrite Planck's equation as follows: Because hc has the value 1240 eV/nm, we can rewrite Planck's equation further as follows: On the basis of these observations, we can plot the relationship between the energy in electronvolts and wavelengths for photons in the visible region of the electromagnetic spectrum (see Figure 2.7). Understanding Light Figure 2.7 The relationship between energy (in eV) and wavelength (in nm) for the visible- light spectrum. COLOR When we consider the components of visible light, our good friend, Roy G. Biv, tells us that there are seven distinct colors. Thus, a logi- cal question you might have, unless you were an art student or a physics major, is how the human eye perceives other colors. The human eye has three receptors that respond to different wave- lengths. In fact, a color triangle popularly used in art and physics classes and which is attributed to the work of James Clerk Maxwell can be used to explain how humans view color. Let's now consider the Maxwell color triangle, which is illustrated in Figure 2.8. Note that the three primary colors (red, blue, and green) are located at the three apices. Most, but not all, colors can be produced by mixing the three primary colors. Maxwell's work paved the way for a more sophisticated color triangle now used to represent all colors. That triangle was developed by the Commission Internationale de l'Éclairage (CIE) in 1931 and postulates three ideal, but nonexistent, primary colors, one located in each corner of the triangle. This forms a display of the amount of ideal blue, red, and green that would have to be mixed to form different colors. This also explains why modern videod