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A reformulated integral equation is solved inside an elliptic disk particle for an electromagnetic field formulation bridging the quasi-static and the physical optics approximations. The scattering amplitude tensor elements associated with such a field formulation are derived and then used to formulate the extinction cross sections. It is shown that the extinction cross sections have a frequency dependence and an incidence angle dependence similar to those associated with the physical optics approximation, and they have a particle shape dependence similar to that associated with the quasi-static approximation. Furthermore, at the high-frequency limits, it is shown that those cross sections could reach the value known in the literature by the extinction paradox, namely, twice the particle geometric shadow area.

The conventional integral equation governing the electric field inside dielectric particles is reformed to bridge and to provide mathematical foundations for analytic techniques widely used to estimate such a field. The solution of the reformed equation inside a dielectric slab explained how inner-field formulations based on the Rayleigh, the Rayleigh-Gans, the quasi-static, and the Shifrin approximations can be supported by the particles. It also confirmed the approach employed to reform the integral equation. The analysis performed uncovered the differences between the depolarization tensor characterizing electrostatic fields inside the particles and the source dyadic resulting from the extraction of the singularity of the integral equation kernel.