Chapter 1 presents a summary of the brief history of the geometric optics method for light-scattering computation. Then, basic physical variables and constants of quantifying light scattering by a particle are introduced. In particular, Maxwell’s equations are briefly discussed to introduce the Poynting vector, which is used in later discussions of electromagnetic energy conservation concerning extinction, scattering, and absorption. We also introduce the concepts of the Stokes parameters, amplitude scattering matrix, optical theorem, phase matrix, and scattering/absorption/extinction cross sections. The simplification of the scattering matrix is discussed with respect to particle orientations in both the particle and laboratory coordinate systems. We also discuss chirality and mirror symmetry associated with light scattering by nonspherical particles. In addition, we discuss the extinction matrix associated with a specifically oriented particle. The remaining portion of this chapter is devoted to the discussion of Snell’s law, Fresnel equations, and Fraunhofer diffraction. The scalar and vector Kirchhoff diffraction formulas are discussed in detail. Furthermore, we discuss the applicability of Babinet’s principle to the diffraction computation of a particle.