1) Shape

diffraction  dif·frac·tion (dĭ-frāk'shən) n.
Change in the directions and intensities of a group of waves after passing by an obstacle or through an aperture.

Whether or not you're a veteran of high school physics, you will recognize that the ocean waves shown here are somehow bending around the rocks.  If you have thought about this process before, you will probably also be aware that the way that diffraction works depends on two obvious things: Wavelength, and the Size of the barrier the waves are interacting with.  Play with this applet to get a feel for wavelength and stuff. 

The bottom line with this simple type of diffraction is that big wavelength waves bend easily around objects, and little ones don't. The best diffraction effects happen when the wavelength and the object in question are about the same size, because that creates interesting patterns of interference - and that brings us to a connection with the nano-world.

Here, you're looking at a scanning tunneling microscope (SEM) image of a razor blade.  It does have roughness at

the microscopic level, but you can see that the ridges are really small - about the same as the wavelengths of light that we can see.  The second thing that's so cool about the razor blade example is that it shows waves interfering, or destroying each other. That's what accounts for the dark fringes in the picture - only waves can do this, and it's key for our big picture.

And Even More Nano:

In the early days of the 1900's, not everyone bought into the idea that salt was bound by ionic bonds, or that X-rays were light, just like the blue light diffracting around the razor blade in the example you just looked at.

When we look at salt using visible light, all we see is white crystal shapes, but that's because visible light is huge compared to salt crystals, and pretty much ignores the salt crystals.  Nope, to see something THAT small, we're going to need some REALLY small waves!

In 1913, Max Von Laue bounced X-Rays off of powdered salt crystals, and found the same type of pattern that you've seen already. You should recognize interference patterns when you see them, and that must mean that Von Laue was finally using light that was in the ballpark of the salt itself!

Just to give you a sense of the sizes here, the sodium atom is about 0.2 nm across.  If you understand diffraction, you're on your way to studying the world of the very small. There is a great deal more to the subject, read on!

Light diffracts around objects, so to see small objects clearly, you need to use small waves of light, because big waves blur and get all messy.

The razor blade photo is one of my all-time favorites, because it shows two things.  First, Razor blades are WICKED SHARP, which means that the blade is smooth to a very fine degree, on the order of a wave of light, or maybe 500 nm.