Understanding colors in nature.

There are two ways in which white light is transformed into colored light by interaction with matter: absorption and scattering. Absorption transforms light into other forms of energy, whereas scattering redirects it. Reflection, refraction, and diffraction are not distinct from scattering but rather are manifestations of it. There is a hierarchy of theories for describing any color phenomenon. For example, many features of rainbows--but not all--can be described satisfactorily by geometrical optics. More accurate descriptions require more exact theories. What one observes when matter is illuminated depends on its disposition. A microscope slide scatters light mostly in two directions. When smashed to bits, it scatters in all directions. Yet its chemical composition has not changed. Glass particles, unlike those in smoke, are usually too large to give colors upon scattering. The colors one sees when smoke is illuminated depend on whether one observes light that has or has not been scattered. Moreover, particle size is crucial in determining what colors are observed. As with scattering, there is a hierarchy of theories of absorption. One can content oneself with an empirical description of absorption or seek a fuller understanding by appealing to microscopic theories. Yet it is sometimes difficult to associate a definite microscopic mechanism with a given absorption feature. Ice is intrinsically blue, yet this cannot be attributed readily to a specific mechanism. Beer has no yellow absorption feature; absorption by it is least in the red and rises steadily toward the blue. Over short paths, beer is yellow; over long paths, it is reddish. Selective scattering by molecules causes the blue of the sky.(ABSTRACT TRUNCATED AT 250 WORDS)