The interaction between light and matter is often very complicated. The general description of the resulting phenomena often uses empirical measurement functions, such as the bidirectional spectral reflectance distribution function (BSRDF) to be discussed in the next chapter. However, the optical properties of a homogeneous material in its simple form (such as gas or crystal) can be calculated from physical principles. Understanding the basic optical properties of material is important because it serves as a foundation for understanding more complex phenomena. In this chapter, we will first discuss the physical properties of light, matter, and their interaction for simple cases. We will then derive the optical “constants” of material that characterize the propagation of light in the material.

Light, energy, and electromagnetic waves

For color imaging applications, light can be defined as the radiant electromagnetic energy that is visible either to our visual system, or to the image capture devices of interest. (When discussing visual systems of different species, we have to vary its range accordingly.) In optics, the scope of definition of light is larger, including other wavelengths for which the behavior of optical elements (such as lenses) can be described by the same laws as used for the visible spectrum. In physical chemistry, light is sometimes used to denote electromagnetic waves of all frequencies.

The electromagnetic spectrum that is visible to our eyes is from about 360 nm to about 830 nm in the air (according to the CIE specifications), corresponding to the frequency range of 3.61 × 1014-8.33 × 1014 Hz.