Abstract Geophysicists commonly recognize that a sizeable portion of a reflector is involved in causing a reflection as seen on a seismic trace, but the areal extent is usually not calculated and hence not appreciated. Most commonly, concepts are simply transferred from classical physical optics and called Fresnel-zone effects. Confusion sometimes results due to the failure to distinguish whether incident waves are plane or spherical. An excellent explanation (but one which does not use the term “Fresnel zone”) is given by Hagedoorn (1954), from which Figure 1 is taken.