A thin slice is cut out of a glass cylinder along aplane parallel to its axis. The slice is placed on a flat glass plate as shown. The observed interference fringes from this combination shall be

The cylindrical surface touches the glass plate along a line parallel to axis of cylinder. The thickness of wedge shaped film increases on both sides of this line. Locus of equal path difference are the lines running parallel to the axis of the cylinder. Hence straight fringes are obtained.