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4.1 Introduction So far, it has been assumed that light propagates as rays. This is not the case. Rays only exist in theory and are a convenient basis for geometric optics. In reality, rays are lines drawn in the direction of the propagation of light. They are the normals to the wavefront in the direction of the flow of the radiant energy. This is indicated in Fig. 4.1, where light radiates from a point source P in spherical waves. After exiting an aberration-free optical system, the wavefront converges precisely toward point P′. If the media through which the light travels are not isotropic, the wavefront is no longer spherical. Advantage is taken of this fact with gradient index material to form the desired wavefront shape. To explain the concept of wave aberration in this fundamental text, we limit ourselves to the discussion of the spherical aberration. It is sometimes desirable to compare the results obtained with geometrical optics. This will be done for third-order spherical aberration. For an in-depth study, the reader is encouraged to consult references 1 and 2. 4.2 Diverging and Converging Waves Light emanating from a point source forms a diverging spherical wavefront. After passing through a fitting optical system, it is converted into a converging wavefront, aiming for the axial image point as indicated in Fig. 4.1.
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