Spheroidal core–mantle particle absorption, scattering, and polarisation in the long-wavelength limit

Context. The numerical calculation of optical properties (extinction, absorption, scattering, and polarisation efficiencies) is often time-consuming for non-spherical and inhomogeneous particles. Where possible, analytical methods are therefore to be preferred. Aims. We provide an analytical tool for deriving the optical properties of mantled spheroidal particles, of arbitrary axis ratios, in the long-wavelength limit (a ≪ λ), where the mantle form may be confocal, co-axial, or of a constant depth with respect to the particle core. Methods. We have developed an analytical approach to spheroidal core–mantle (CM) particle optical property calculations. Results. The analytical method compares well with DDSCAT numerical calculations and, under limited circumstances, with those made using the Bruggemann effective medium theory (EMT). Conclusions. The analytical method presented here provides a useful tool for exploring the optical and polarisation properties of CM spheroidal particles at long wavelengths (λ ≳ 8 μm) and is simpler and faster to implement than corresponding numerical methods. We caution against the use of EMT methods in approximating the optical properties of CM particles.