Hi!
I'm interested about this optical phenomenon but don't know how to explain why wavefront become more and more circularity when the slit is reduced?
http://www.shrani.si/?1Z/OG/2OhyYDXX/1/fizf0258.jpg
http://www.shrani.si/t/2k/wf/wMxtVum/1/fizf0260.jpg