the equivalent of a transmission hologram of a converging lens. For a smooth zone plate, the opacity (or transparency) at a point can be given by: where is the distance from the plate center, and determines the plate's scale. Binary zone plates use almost the same formula, however they depend only on the sign: Free parameter It does not matter to the constructive interference what the absolute phase is, but only that it is the same from each ring. So an arbitrary length can be added to all the paths This reference phase can be chosen to optimize secondary properties such as side lobes. Applications Physics There are many wavelengths of light outside of the visible area of the electromagnetic spectrum where traditional lens materials like glass are not transparent, and so lenses are more difficult to manufacture. Likewise, there are many wavelengths for which there are no materials with a refractive index significantly larger than one. X-rays, for example, are only weakly refracted by glass or other materials, and so require a different technique for focusing. Zone plates eliminate the need for finding transparent, refractive, easy-to-manufacture materials for every region of the spectrum. The same zone plate will focus light of many wavelengths to different foci, which means they can also be used to filter out unwanted wavelengths while focusing the light of interest. Other waves such as sound waves and, due to quantum mechanics, matter waves can be focused in the same way. Wave plates have been used to focus beams of neutrons and helium atoms. Photography Zone plates are also used in photography in place of a lens or pinhole for a glowing, soft-focus image. One advantage over pinholes (aside from the unique, fuzzy look achieved with zone plates) is that the transparent area is larger than that of a comparable pinhole. The result is that the effective f-number of a zone plate is lower than for the corresponding pinhole and the exposure time can be decreased. Common f-numbers for a pinhole camera range from to or higher, whereas zone plates are frequently and lower. This makes hand held shots feasible at the higher ISO settings available with newer DSLR cameras. Gunsights Zone plates have been proposed as a cheap alternative to more expensive optical sights or targeting lasers. Lenses Zone plates may be used as imaging lenses with a single focus as long as the type