Question

What are non localised fringes

Answer 1

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Nonlocalized fringes exist everywhere within an extended ( three-dimensional) region of space, for example, in Young's double slit experiment, the fringes fill the space beyond the secondary sources with a whole array of real fringes. This means that these fringes are literally nonlocalized, in the sense that they are not restricted to some small region of space. These kinds of fringes are generally produced by small sources, like point or line sources. In contrast to this, localized fringes are clearly observable only over a particular surface, whether near a thin film or at infinity. These types of fringes result from use of extended sources but can also be generated by point sources as well. Pohl interferometer is useful to illustrate these two types of fringes since with a point source it produces both real nonlocalized and virtual localized fringes.