5.1 Importance of Physical Optics Physical optics defines the transport of light by the physics of wave propagation. Maxwell's equations and Fourier transformations are applied toward accurate descriptions of point spread and defocus. Comprehension of the ensuing topics is essential for an effective design of an optical instrument. Development of comprehension is a product of exploration and application of these principles. Refraction, reflection, and scatter are all based on the wave nature of light. The Airy pattern is dependent on the diffraction of a circular aperture, and the depth of focus is dependent on physical optics. Most sections of this chapter have a more detailed version in Chapter 19, which focuses on advanced concepts. At least one member of the optical design team should comprehend the advanced concepts. 5.2 Wave Equation The wave equation in Chapter 19 [Eq. (19.1)] may be reduced to a simple expression for the spatial frequency k and the temporal frequency ω: k2 = eμω2, where e is electric permittivity and μ is magnetic permeability. The electric permittivity is positive for a dielectric and negative for a conductor. The spatial frequency in radians per distance is k = 2π/λ, where λ is the spatial period. The temporal frequency ω in radians per time is expressed as ω = 2π/T, where T is the temporal period.