One of the most obvious visual aspects of light is the phenomenon of color. Yet color, for all its vivid sensation, is not inherent in the light itself. Color is a perception, not a physical quantity. Color is associated with the wavelength of light, but the fact that we see light with a wavelength of 650 nm as “red” tells us how our visual system responds to electromagnetic waves of this wavelength. There is no “redness” associated with the light wave itself.
Most of the results of optics do not depend on color; a microscope works the same with red light and blue light. Even so, indices of refraction are slightly wavelength dependent, which will be important in the next section where we consider the resolution of optical instruments. And color in nature is an interesting subject, one worthy of a short digression.