Let us calculate this integral for simple geometries.   (Fig. 7c) See the figure presented above. All four surfaces marked as ‘AB' have the same projected area, marked as A'B', on Y-Z plane because the depths (h1 and h2) of A and B are the same for all four geometries. Therefore, will assume identical values for all cases as follows:                (Assuming unit width of the surface)   (Fig. 7d) Let us re-arrange the right hand Side of the expression