We do optical simulation to understand the behavior of optical systems, along input and output parameters that matter to the goals of the system. Suppose you w

We do optical simulation to understand the behavior of optical systems, along input and output parameters that matter to the goals of the system. Suppose you want to make a telescope with a certain spec for magnification and resolution. Or, suppose you wish to design a headlight for a car with bright and even illumination over a given angular range. Or, imagine you need to specify an interferometer for precision displacement measurement at the nanometer level. For each of these systems, there are first principles equations and approximations that describe ideal and slightly-different-from-ideal behavior, but for anything other than very simple optical systems, these first principles approaches become unwieldy very fast. An optical simulation represents the physics of the design and approximates the output you would measure when you build the system. This allows you to optimize and perturb the design over manufacturing tolerances at the design stage, rather than iterating through these at the manufacturing stage (a rather impractical alternative). The capability of any simulation approach depends on the phenomenon you need to model, and the degree of abstraction appropriate to that phenomenon. An “exact” physical representation of an optical system essentially entails solving Maxwell’s equations along the initial and boundary conditions of the optical system. There are times when this is both necessary and practical, but (outside of semiconductor applications) that tends to be rare. Ray tracing, which you mentioned, is a very popular and useful approximation for imaging and illumination. It’s especially useful when the wave properties of light are either not very relevant or they are very well behaved. Other simulation techniques incorporate more of the wave nature of light: Fourier techniques, field tracing, coupled waves, finite difference time domain, etc. It takes some domain knowledge in the application space to know which to apply.