A fresnel is a unit of frequency equal to one trillion cycles per second (i.e., 10<sup>12</sup> s<sup>âÂÂ1</sup>). It was occasionally used in the field of spectroscopy, but its use has been superseded by terahertz (with the identical value 10<sup>12</sup> hertz). It is named for Augustin-Jean Fresnel the physicist whose expertise in optics led to the creation of Fresnel lenses.