Huygens-Fresnel principle and Diffraction

[Jurif]

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

 

[derek e]

You can check out Introduction To Modern Optics by Fowles for a classical mathematical description of the phenomenon. This book casts Huygen's principle into the Fresnel-Kirchhoff Formula and then applies it in several settings. If you think about it, shrinking the aperture localizes the disturbance to a "point-like" area. When the disturbance then propagates (towards the left in the picture), the "point-like" area bothers all of its surrounding neighbors, which bother their neighbors, etc...; symmetrically. It is like dropping a pebble in a pond and producing a circular wavefront.