Figure 3.7 — The optical theorem Figure 3.7 — The optical theorem. The optical theorem. The theorem [Eq. (3.36)] relates the extinction cross-section to the forward scattering amplitude, i.e., the amplitude of the field scattered in the direction of the incoming electromagnetic wave. In this way, the optical theorem permits one to quantify the power removed from the incoming wave. The square surfaces represent the area of a detector that moves towards infinity along the z-direction and, at the same time, becomes larger but at a lower rate: eventually this detector will occupy the whole xy-plane, but it will subtend an infinitesimal solid angle.