A Young’s double slit experiment uses a monochromatic source. The shape of the interference fringes formed on a screen is:

ShutterstockIn Young's double slit experiment (YDSE), if the distance between the slits and the screen is large, the locus of points with a constant path difference on the screen are hyperbolas (since the slits are point sources in 3D).However, for a screen placed perpendicular to the bisector of the slits at a large distance, these hyperbolas appear as straight lines in the central region of the screen. Standard textbook convention for "shape of fringes" in YDSE is straight lines.