The wavelengths of concern in photolithography (on the order of 10 -7 m ) are small compared with most physical dimensions. In situations such as transmission through lenses, we can approximate the propagation of light by neglecting the finiteness of its wavelength. In this chapter we derive that in the limit of zero wavelength, Maxwell's equations give rise to optical laws that can be formulated in the language of geometry. Light energy is transported along rays that are orthogonal to geometrical wavefronts. Optical phenomena can be deduced by determining the paths of light rays and their intensities.
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