My email response to a request for more information about Fraunhofer and Fresnel diffraction:


Let me see if I can give it a shot, but I recommend also looking at diagrams in the books I mention at the end:

Diffraction is a phenomena arising from the wave nature of light. A diffraction pattern appears because light will interfere with itself, just as waves do. But the difference between Fraunhofer and Fresnel diffraction is dependent on how you setup your experiment.

Fraunhofer diffraction: Light can occur as plane waves, which we can imagine as the waves that come rolling in over the ocean. These plane waves can hit an obstruction, like when ocean waves hit a dock, and travel on in a very different pattern. At a large (compared to the size of the obstruction) distance away from the obstruction, there will be an illumination pattern of light and dark depending on the direction from the obstruction. This pattern is a Fraunhofer diffraction pattern. The (effective) source of light and the place where you're recieving the light must be relatively _far_ from the obstruction (e.g. >1 meter from a 0.1 millimeter slit or hole), hence the alternate name of "far-field" diffraction. These distances must be large enough so that the light that arrives and leaves the obstruction and reaches the wall or screen are nearly plane waves.

(An experimental detail: a common source is for this experiment is a laser, which inherently produces plane waves, so the laser itself can be placed near an obstruction, but the screen must still be far enough away. And there are other tricks with lenses one could do.)

Fresnel diffraction: Light can also occur as spherical waves, which is analogous to the circular waves expanding from where we just dropped a pebble in water. A point source of light produces spherical waves. After these spherical waves pass by an obstruction, they will produce a Fresnel diffraction pattern at the screen or wall where they arrive. In general, the distance between the source and obstruction and the distance between the obstruction and the screen can be arbitrarily close, but the interesting distances are on the order of centimeters to 1 meter from a millimeter sized obstruction. Because these distances are not too far, this phenomenon is also called "near-field" diffraction.

(An experimental note: A device called a "spatial filter" can convert a laser's light into spherical waves. It's a technical sounding name, but all it really is is a lens and a very small hole. The lens focuses the laser light onto the hole, and the hole only lets through light from a _very_ small volume of space, so the hole essentially becomes a point source of light, and violá, spherical waves.)

On the theoretical side, computing a Fraunhofer diffraction pattern was traditionally much easier than computing a Fresnel diffraction pattern. As a result, most textbooks only gloss over, if not overlook, Fresnel diffraction and leave the impression that only one kind of diffraction exists.

I take it that you have seen the images on my Fresnel web page, so now you have an idea of what Fresnel diffraction patterns look like. You can probably get more information from your nearest large public or university library for optics or diffraction type stuff. One good physics book is _Physics_ by David Halliday and Robert Resnick (there have been so many editions it seems; most of them are good). It has very nice chapters on waves, light, and interference, and it has a chapter mostly devoted to Fraunhofer diffraction. An excellent book that looks at the two diffraction types in an even-handed way is _Optics_ by Eugene Hecht (2nd ed.). That book has many diagrams, examples, explanations for both types of diffraction.

And, of course, if you want to make your own Fresnel diffraction patterns, download my program. Although I don't specifically know where, I've heard there are lots of programs out there that do Fraunhofer diffraction* (or just Fourier transforms).

I hope this helps,

Dean Dauger

* Note: The Fresnel Diffraction Explorer now does Fraunhofer diffraction too, as of version 1.1.