
10
COHERENCE
In this chapter, we address how to describe superposition of two or more light waves. This
issue arises in Chapter 7 where we combine two waves at a beamsplitter as in Figure 7.1A,
or an innite number of waves in a Fabry–Perot interferometer in Section 7.5. The question
of superposition arises again in Chapter 8 when we integrate contributions to the eld in an
image over the pupil in Equation 8.32, and implicitly in geometric optics through Chapters 2
through 5, when we combine rays to form an image, the details of which we will address in
Chapter 12.
Because Maxwell’s equations are linear partial differential equations for the eld