A ray is represented by a vector: a straight line indicating the magnitude and direction of propagation. A wavefront is a notional surface locally normal to a ray. Thus, a wavefront could be a plane (all rays parallel, as if from infinity) or a curved surface (indicating diverging rays, as if emanating from a point). Figure 2.1 illustrates rays and wavefronts for both cases.
A beam can be defined by two separated elements of area as shown in Fig. 2.2. It is thus the locus of possible rays that pass through the two areas separated by distance d. As dA1 and dA2 approach zero, the beam approaches a single ray. Since there is a small, but nevertheless finite, cross-sectional area associated with a beam, it is capable of carrying power without the flux density (power per unit area) approaching infinity.
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