Doron pasted a diffraction pattern resulting from a perpendicular setup, like this one:
http://micro.magnet.fsu.edu/primer/java/diffraction/basicdiffraction/
So I simply stay with the basic concept. You can see that the answer to one of the principle inquiry into the diffraction effect doesn't include the frequency, namely the position of secondaries where
sin(angle) = wavelength/distance. When the subject involves this topic
http://transition.fcc.gov/mb/audio/fmq.html
the wavelength doesn't come often to the surface even though frequency and wavelength are closely related, as you said.
The laser beam is a well-suited medium to study the principle effects of diffraction. Which one would you chose?
http://www.laserglow.com/S53
There is a variety offered, such as 435 nm Blue, 501 nm Green, 556 nm Yellow, and so on. The manufacturer uses the wavelength, not the frequency to make the distinction and that's because of the assumed application of the device.
[qimg]http://www.lbl.gov/images/MicroWorlds/EMSpec.gif[/qimg]
If you want to study proteins, then ordinary microscope won't do -- you need to buy one that emits EMR of a wavelength that is shorter than the one of the visible light, but the ideal condition of perpendicularity wouldn't materialize under these conditions. So other consideration has to be taken into account, but none of them concern the one that you have been discussing with Doron. Maybe I was late when the shift toward "single and singular frequencies" left the topic of diffraction and entered another one. Only Doronetics can provide a seamless transition from single and singular frequencies to negative dimensions of objects (measured in Hertz per dimension, I guess.)