In this paper the properties of caustic patterns necessary to apply the caustic experimental method to stress analysis in cutting operations were made clear. Under simplified two dimensional conditions, the object of study is a wedge shape plane with discontinuously distributing oblique load divided into 2 or 3 parts over loading region. Specimens are made of acrylate resin plate of about 3 mm thickness. The caustic patterns formed by reflected lights from the both surfaces of plate are observed and those characteristics with loading conditions are investigated in detail. On the other hand the computational patterns based on optical theory and mapping method are plotted. These two patterns are coincided accurately enough. With gained results the convenient simple formula to evaluate one end vale of external load is proposed.