Monochromatic waves traveling

Let us consider an isolated molecule perturbed by an electromagnetic field. According to the semiclassical approach, the external radiation is described as a plane monochromatic wave traveling with velocity c and obeying the Maxwell equations [21] (i.e., the fields are not quantized). 

The assumptions of the kinematical theory - that the incident wave is monochromatic and plane and that there is no absorption of either the transmitted wave or the scattered waves - are reasonable assumptions with which to begin a theory. However, the other assumptions - that a scattered wave is never rescattered and that there is no interaction between the transmitted and scattered waves - are gross oversimplifications of the physical situation. For example, consider a plane monochromatic wave incident upon a crystal plate at such an angle that the Bragg law is satisfied for a set of planes approximately normal to the crystal plate, as shown in Figure 3.20. It is clear from this diagram that at A the diffracted wave Si in the crystal is rediffracted so that it travels in the same direction as the transmitted wave T. This rediffracted wave is denoted S2. There is no reason why S2 should not be also rediffracted at B to produce the wave S3, as shown. Thus, the assumption that the diffracted wave, once produced in the crystal, is never rediffracted, is clearly unacceptable. 

Retardation Techniques. Various optical systems have been used for this purpose. The most common is the polarized light microscope equipped with a compensator. Usually, the in-plane birefringence A/ii2 (the refractive index difference between principal axes lying in the plane normal to the axis of the microscope) is measured (Fig. 4). The monochromatic wave of light divides into two parts, polarized along the 1 and 2 principal directions which travel at different velocities ui and U2. ... 

The plane traveling wave is monochromatic if the source of oscillations is harmonic. Eqs. (2.8.5) and (2.8.7) describe monochromatic waves. In linear media this corresponds to the fixed wavelength. 

Let us assume an active medium that responds to the energy-level diagram of Figure 2.6(a). It consists into four energy levels E, with respective population densities M (i = 0,..., 3). Let us also assume that laser action can take place due to the stimulated emission process E2 Ei. When a monochromatic electromagnetic wave with frequency v, such as (E2 — E )lh = v, travels in the z direction through the medium, the intensity of the beam at a depth z into the crystal is given by... 

Even with completely monochromatic light, pulse spreading can still occur, because the radiation can take various paths, or modes, through the fibre, as sketched in Figure 14.31. It is apparent that a ray that travels along the axis of a fibre will travel less than one that is continually reflected on its journey. [In fact, the dispersion that results cannot be properly understood in terms of the transmission of light rays, and the various modes are better described in terms of the allowed wave patterns that can travel down the fibre.] The resultant pulse broadening, due to the various modes present, is called modal (or intermodal) dispersion. In order to overcome modal dispersion a number of different fibre types have evolved. 

A wave of monochromatic, plane-polarized light may be considered to be the resultant of two vectors a wave of right circularly polarized light and a wave of left circularly polarized light. A medium that is optically active has different indices of refraction for these two components. As a consequence, the plane of polarization of such light will be rotated as it travels through an optically active medium. The specific rotation of an optically active substance is defined as... 

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