Moire fringes are observed when-

Explanation:

• In the simplest and most familiar case, moire fringes are observed when two parallel, fairly coarse bars and slit gratings of different pitches are held between the eye and an extended source.
• Or, Moire fringes are observed when Index grating is moved over scale grating.
• When the gratings overlap, the wide bars shown in the below figure appear.

• Superimposed gratings of the equal pitch will also produce a moiré pattern when given an angular displacement about the normal to the plane of the gratings, as shown in the figure below.

​

• There are many types of gratings whose fringes cannot be observed this way, especially if the gratings are of a fine pitch.
• Moreover, moiré patterns containing contributions from both of these basic types are often encountered, as well as complexities arising from grating irregularities and angular displacements other than those mentioned previously.
• The designation, moiré, is derived from the effect produced by the fabric of the same name when two layers are in contact.