F is a flux of the first sort, ∫F·dS is a flux of the second sort (through surface S). dS is the differential vector area element, n is the unit normal to the surface. Left: flux (type 2) is proportional to the component of flux (type 1) that flows normal to the surface. Right: The reduction in flux (type 2) through a surface can be visualized by reduction in F or dS equivalently (resolved into components, θ is angle to normal n).