TABLE OF CONTENTS Except for our treatment of polarization (which is a fundamental vectorial property of electromagnetic waves), we have regarded the propagation of light in geometrical terms. Hence we have treated light as a form of energy propagated in straight lines. We now acknowledge three other properties of light that are explicitly dependent on its wave nature: coherence, interference, and diffraction.
As described previously, the propagation of electromagnetic waves is described by a second-order partial differential equation [Equation (4.1)] so that the principle of linear superposition holds. We can immediately exploit this principle in describing coherence by considering two sources of plane harmonic linearly polarized waves. The fields produced by these two sources are given by
If the initial phase difference between these two sources, ? 1 ? ? 2, is constant, then the sources are said to be mutually coherent. Under these circumstances if we examine the intensity of the total field at some point r from the sources we obtain
or
where ? = ( k 1 ? k 2) r + ( ? 1 ? ? 2), and I 1 = E 1 2 and I 2 = E 2 2.
The interference term 2 E 1 E 2 cos ? contains spatially periodic intensity variations because ? depends on r. If the sources are mutually incoherent (and at the same wavelength),...
