SEARCH Articles Figures Tables A sketch of the solid angle in the special case when the arbitrary surface coincides with the unit sphere. In this particular case n e, r r 1, hence from it follows that dfi dA. [Publishers Imprint] See Also: A sketch of the solid angle in the special case when the arbitrary surface coincides with the unit sphere. In this particular case n e, r r 1, hence from it follows that dfi dA.