So far everything we have done has considered functions of only one independent variable, namely f(x). However, in much of optics, we have to deal with functions of two spatial variables, for example f(x, y). In this course we are only going to work with two coordinate systems. Primarily we are going to consider rectangular coordinates, i.e. x and y. However, occasionally we will also consider polar coordinates, g(ρ, φ). We will strive whenever possible to identify the coordinate system where our function is separable.