Possibility of making an experiment in a classroom to simulate DNA diffraction I am a TA in a structural chemistry class. The professor want me to show students how Watson and Crick determined the structure of DNA from X-ray diffraction results of DNA crystals. The professor suggested me to print some sine functions in a paper and let visible laser beam from a laser pointer to go through the sin functions to get an "X" shape diffraction pattern, just like the X-ray diffraction pattern of DNA. 
I just wonder is it possible? 
I think the X-ray diffraction of DNA is similar to grating diffraction. And the grating constant should be in the order of microns to be able to significantly diffract visible light. Is my professor kidding me? Or is there any possible ways for me to simulate diffraction with a paper and a laser pointer in a classroom?
 A: Reflection diffraction gratings can be made very easily from compact discs (CDs).
You get a diffraction pattern simply by reflecting a laser pointer from a CD. If you know the wavelength of the pointer you could work out the groove spacing. Someone has kindly provided a lab script for that very experiment. Or see this one.

If you want to do more than demonstrate diffraction patterns and build a spectrograph using a CD grating and a cereal box - here are the plans. These work a charm and can be used to look at arc lamps, fluorescent lights etc.
A: I'm not sure a 2d structure will give you the required phase information unless you can compute that before printing, in which case you are making a hologram. What about some 3d structure that is on the same scale? I don't know what the finest incandescent bulb filament is you can acquire, but something like that should work. Also, traditional crystallography requires repeating arrays to improve resolution.
Here is an example of what can be done with a stretched filament and an ordinary laser:

A: For what it's worth, you might want to look up the original paper on the theory of helical diffraction: "The structure of synthetic polypeptides. I. The transform of atoms on a helix"
W. Cochran, F. H. Crick and V. Vand, (1952) Acta Crystallographica 5, 581-86.  This is the seminal work that allowed Crick and Watson to deduce the DNA structure.
It is not a simple paper, but it does show how the 'X' diffraction pattern from a helical fiber (not a crystal, btw, since it's not ordered in 3 dimensions) arises in terms of Bessel functions.
Still, despite your fine efforts in getting laser diffraction to work, it is not obvious to me that the pattern from a helix, which is three-dimensional, can be gotten from a two-dimensional object.  Does your professor have the theory worked out in detail?  That would be awesome. 
