The simplest quantitative understanding of light is facilitated by geometric theory of light also known as Ray/geometric optics. The theory is based on a set of postulates that are found to preserve everyday observations on length scales much longer than the wavelength, and are not violated except under special circumstances. Among the postulates, the most profound being Fermat’s principle which governs the propagation of light traveling between two points. Specifically, it states that light takes minimal time to travel between two points, and largely the path of minimum time is also the the path of minimum distance (Hero’s law). Fermat’s principle can be taken as the governing principle to derive the core laws of ray optics i.e, law of reflection, and law of refraction (Snell’s law). This opens the door for predicting the path of light as it propagates through interfaces and key elements of (e.g., mirrors, optical glasses, lenses etc) an optical system. The theory also forms the basis for some of the celebrated simplistic formulas (imaging equation and lens-maker’s formula). Moreover, this leads to accurate functioning of complex optical imaging components such as, beam-expander and objective lens. In addition, we will discuss different aberration effects caused by these optical components. The chapter also discusses some of the the optical components and systems necessary for constructing a high-resolution precision optical microscopy system.