Laser is parallel light, but due to the diffraction effect of a single slit, light can spread in all directions and can be imaged with or without convex lens. The function of convex lens is to facilitate theoretical calculation, just like Young's double-slit experiment. There is no convex lens in the book, but the double slit is the simplest grating, and a convex lens should be added. If there is no convex lens, many approximations will be used in calculating the fringe spacing. For example, the opening angle of the fringe relative to the center of the double slit is approximately equal to the diffraction angle, but for convex lenses, it is not approximately equal, but strictly equal.