[UPSC CSE 1996] Diffraction (Q289, 20M)

A fine slit is illuminate by monochromatic light of wave-length 6000 \text{\AA}. A thin wire is placed parallel to the slit and the diffraction pattern is observed on a screen at a distance of 1 m from the wire. In the shadow of the wire equidistant fringes of thickness 1.5 mm are observed. How do you explain this observation ? Calculate he diameter of the wire also.