# Electron diffraction experiment

1. Nov 27, 2014

### Coffee_

I realize this is a very basic question, but I've been thinking about this for a bit and can't seem to find an answer:

When I did the electron diffraction experiment on graphite, I saw 2 rings on the fluorescent screen. The description of these rings should be given to me by Bragg Law, namely $2dsin(\frac{\theta}{2})=n\lambda$. I'm inclined to take $n=1$ for the first ring and $n=2$ for the second ring to the analogy of classical optics experiments but then I realize I'm not sure what I'm doing.

I don't know how the planes are oriented in the graphite plate, it could be possible that this second ring is just diffraction happening of a different set of planes and such that $n=1$ for the second ring as well. The other things I'm doubting about is that if I'd received only one ring, how could I tell which $n$ this ring corresponds to.

As you can see from this post I'm pretty confused. I need to understand this for the experiment but I don't really have the time right now to start learning the deep theory behind this phenomenon. I looked around online and all the descriptions are either to shallow or either to complicated. So I'd be very grateful if someone can read this and try to elaborate a little about these $n$. Again I know the basic derivation of Bragg Law, but I have trouble with fully connecting this law to the rings I see.

2. Nov 27, 2014

### Staff: Mentor

The area of the graphite that the beam passes through contains many many tiny crystals, with their planes oriented in random directions. Only those crystals whose planes are oriented at a certain angle to the beam can produce the diffraction that generates a particular ring. These crystals have different azimuthal orientations around the beam direction (if you know about spherical coordinates, think about the angles θ and φ), and this gives you the circular ring.

3. Nov 28, 2014

### Coffee_

This would mean that n=1 for the second ring as well right?

4. Nov 28, 2014

### Staff: Mentor

Yes. the two rings are produced by different sets of planes (in the same crystal structure) with different spacings. You're seeing the n=1 diffraction order in both cases.

A Google search on "electron diffraction crystal planes" turned up the following diagram. The red and blue lines show the relevant planes, with spacings d1 and d2.

Last edited: Nov 28, 2014
5. Nov 28, 2014

### Coffee_

Thanks a lot for your time, it's getting more clear now. Do you mind if another question comes up over the coming days I'll just post it in reply?

6. Nov 28, 2014

### Coffee_

Why do I only find d1 and d2 by doing this experiment? At first glance it seems to me that much mroe orientations are possible. One for example would be the distance between the planes. Just looking at the picture I can see two sets of plane that differ from d2 and d1 in distance.