We already have discussed at length the fact that electromagnetic theory is basically linear as long as all materials involved behave in a linear fashion. The whole theory of diffraction and the propagation of optical fields was based on this property of light waves. The present chapter studies the consequences of this property from yet another point of view. In some sense we treat a special case of the subjects discussed earlier, but it sheds new light and provides several interesting applications.
In this chapter we shall be mainly interested in some practical aspects of the interference effects occurring with coherent waves and, accordingly, we shall assume that all fields are coherent in time and space in the sense discussed in chapter 7. Moreover, we still assume the validity of the scalar approximation. If this approximation does not hold the situation becomes more complicated, as will be discussed in chapter 9.
Interference effects are exploited in science and technology for various applications. We shall see in chapter 11 that the superposition of two coherent waves can be used to record a complete wavefront with amplitude and phase as a hologram. A hologram can be also interpreted as the record of the interference pattern produced by this superposition. In this chapter we introduce the family of instruments based on interference effects, namely, interferometers. Following a discussion of the general aspects of interferometry we describe several interferometers in detail.
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