The condition for observing Fraunhofer diffraction from a single slit is that the wavefront incident on a single slit must be

Step-by-step solution

In Fraunhofer diffraction, the source of light and the screen are effectively at an infinite distance from the diffracting slit. As a result, the wavefronts of the light incident on the slit are plane wavefronts. This is typically achieved by placing the light source at the focus of a convex lens, which converts the diverging spherical wavefronts into plane wavefronts before they strike the slit. Thus, the incident wavefront must be plane. Answer: plane.