The physical basis of hyperspectral remote sensing is the presence of features in the observed spectral properties of materials that are characteristic to their chemical and physical make-up. To establish this physical basis, one must explore the interaction of optical radiation with matter to understand how material properties are expressed in radiometric observations. Since materials are composed of atoms and molecules whose physical understanding requires a quantum mechanical treatment, a full understanding of the spectroscopy of materials is derived from quantummechanical principles. However, classical electrodynamics offers several useful insights into the underlying physics and provides foundational principles of significant utility with regard to the remote sensing field.

This chapter reviews the fundamental principles of the interaction of optical radiation with matter that arise from the classical theory of electromagnetic radiation. The focus of the treatment is interaction and propagation through dense media, and the characterization of such media by a complex index of refraction. It is shown that the complex index of refraction is an intrinsic spectral property of the material that governs the transmission and reflectance at material interfaces, as well as transmission through uniform media. A classical model for the complex index of refraction that can be further extended by quantum mechanics relates the spectral locations and shapes of resonant spectral features to the electrical and mechanical characteristics of the atoms and molecules from which the material is composed.