----- Original Message ----- From: <erick...@hickorytech.net> To: <pinhole-discussion@p at ???????> Sent: Wednesday, December 11, 2002 9:27 PM Subject: Re: [pinhole-discussion] What is Diffraction?
> Here's a question that has troubled me for a long time, since we've gained > the attention of the physicists among us. If the optimal image occurs when > all light waves are 'in phase", which the Young article says occurs at the > junction between the nearfield and farfield diffraction patterns (whatever > that is), is there another point further on where the various waves again > come into simultaneous phase sync, and thus optimal sharpness? Theoretically > there should be, but how far? Physicists analyze diffraction using either near or far field methods. Far-field means the source of light is far relative to the aperture size, the wave front arriving at the aperture is assumed to be flat. Near-field is the apposite, wave front is assumed to arrive curve shaped. This, I believe, also affects or determines how far from the aperture the near-field diffraction reaches and how far from the aperture the far field diffraction starts. There is a gray area in between, this area can be assumed to separate near and far field diffractions voiding any "juncture" of them, and as Young's article says: "here, the image is not amenable to description by simple arguments". When you talk about "in phase" you may be thinking about how a zoneplate works, where the aim is carefully position the edges of the clear rings so the diffractions caused by all of those edges positively interfere at the focal point and therefore add up their intensity. Pinhole does not work like that. For my view of how zoneplates work, pls read: http://members.rogers.com/penate/zoneplate.html It talks a bit about the near and far field diffraction, also. Don't quote me on all this pls, this is just a layman's view of the topic. Guillermo
