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When an optical flat is placed on another surface and illuminated under mono chromatic light source, the light waves reflect off both the bottom surface of the flat and the surface it is resting on. The reflected waves interfere, creating a pattern of interference fringes, visible as light and dark bands, The spacing between the fringes in smaller where the gap is changing more rapidly, indicating a departure from flatness is one of the two surfaces, in a similar way to the contour lines on a map. A flat surface is indicates by a pattern of straight, parallel fringes with equal spacing, while other patterns indicate uneven surfaces.
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