1. The portion of a reflector from which reflected energy can reach a detector within one-half wavelength of the first reflected energy. The central first Fresnel zone is elliptical (circular for zero offset data) for a horizontal reflector; successive Fresnel zones are annular rings of successively larger radii; see Figure F-25. Most of the energy reflected from the first Fresnel zone interferes constructively but the outer portion does not contribute much, and the effective size of the Fresnel zone is smaller by about a factor of √2. Because the size of the Fresnel zone is frequency-dependent, the dominant frequency is often used where a spectrum of frequencies is involved. 2. The first Fresnel zone only, the portion of a plane reflector mainly effective in generating a reflection since, for a harmonic wave, the effects of successive zones generally cancel each other. A nomogram for determining the radius of the first Fresnel zone is shown in Figure F-26. See Sheriff and Geldart (1995, 152). 3. A Fresnel lens has alternate zones blacked out so that only constructive zones pass light.