Cirrus clouds have a large effect on the atmospheric system and radiative budget of earth as theyhave a huge influence in all scales of weather processes and climate. Cirrus clouds are composed ofnon-spherical ice crystals with various shapes and cover about 30%-40% globally. The appropriategeometric models (applied mathematics and computational geometry) and incident coordinatesystems determine the shapes of ice crystals [1]. The shapes include but not limited to: hollowcolumns, dendrites, bullet rosettes and capped columns. The scattering solution however inthe infrared's part of the spectrum is a complex problem. There are established methods thathave been used to solve scattering methods such as Mie, T-matrix and Rayleigh theories. Thesemethods are not efficient at infrared wavelengths due to size parameter, which typically can bein order of 100 or more that leads to intensive computation. In this work, a new approach basedon physical optics is presented in which the scattering by smooth prismatic dielectric ice crystalswhich are electrically large is evaluated. In this method, the total scattered amplitude in anydirection can be evaluated by application of the reaction theorem to a volume interval in principlefrom a knowledge of the internal field distribution. This integral in turn can be converted to anintegral over the equivalent surface current distribution. The method enables the evaluation of thetotal surface field. The scattered field solution of the proposed physical optics method (PO) hasbeen compared with ray launching geometrical optics (RL-GO). Overall, the proposed methodshows a good agreement when the size of the scatterer is large compared to the wavelength. Theproposed method has a good potential to be applied for ice crystals with rough surface structure.