The physical phenomenon called diffraction is of the utmost importance in the theory of optical imaging systems.
—J. W. Goodman 
The optical field is an electromagnetic field. The physical nature of the field is determined by the laws of electromagnetic propagation and by quantum mechanical and thermal laws describing the interaction between the field and materials. In the design and analysis of optical systems we consider
While we do not consider light generation in detail in this text, differences in the coherence properties of the source are central to our discussion. Coherence theory, which relates the electromagnetic nature of the field to statistical properties of quantum (e.g., photonic) processes, is the focus of Chapter 6.