Figure 4.6 shows how the wavelengths are distributed for common light sources. An ideal light source would have all visible wavelengths represented with equal energy, leading to idealized white light. The opposite is total darkness, which is black. We usually do not allow a light source to propagate light directly onto our retinas (don't stare at the sun!). Instead, we observe light that is reflected from objects all around us, causing us to perceive their color. Each surface has its own distribution of wavelengths that it reflects. The fraction of light energy that is reflected back depends on the wavelength, leading to the plots shown in Figure 4.7. For us to perceive an object surface as red, the red wavelengths must be included in the light source and the surface must strongly reflect red wavelengths. Other wavelengths must also be suppressed. For example, the light source could be white (containing all wavelengths) and the object could strongly reflect all wavelengths, causing the surface to appear white, not red. Section 6.3 will provide more details on color perception.
Steven M LaValle 2020-01-06