Effects of Tangent Plane Approximation and Divergence Factor in Light Scattering by a Large Dielectric Object with Curved Surface

For research on the electromagnetic wave scattering by large objects/particles, the tangent plane approximation is often used with Physical Optics (PO) due to its efficiency and accuracy, like scattering of the airplanes, light scattering of an air bubble in water. Because the curvature radii of the surface of these objects are large compared to the wavelength, the tangent plane approximation is applied to describe the shape of the objects. The contour of the object is therefore organized by lots of discrete tiny facets, thus, on each of these facets, the incident field and refracted (or reflected) field are considered as plane waves. Consequently, when there are multi-interactions between the object and the wave, the convergent or divergent character of the wave is omitted (or considered as constant 1). However, according to our study for the scattering of a dielectric spherical and a cylindrical particle, there are obvious discrepancies of the scattering field between PO and rigorous Debye theory. As the divergence factor is considered in Vectorial Complex Ray Model (VCRM) for the multi-interactions of the wave with particle surface, the scattering fields calculated by combining method VCRM+PO and Debye theory are in good agreement. In this paper, we extend the tangent plane approximation to the scattering of a large dielectric object and examine the effect of the plane approximation by comparison with the hybrid VCRM+PO method and Mie theory in scattering of a sphere and infinite circular cylinder. We found that for scattering of a large object with curved surface, the effect of divergence factor of the wave should be considered for the multi-interaction processes. The further research for scattering of a more complex object is under study.