# Trying to understand electron diffraction

I'm trying to understand how electron diffraction works, but I'm not really getting it. Here's an image

Here are a couple of questions about it that I can't figure out.

Why is the angle of the beam $2\theta$ instead of just $\theta$?
Why are there exactly two beams (outer and inner) when the experiment is performed? I would expect there to be several that got dimmer as they went out from the center.

If someone could explain the concepts in basic language to me, including the specific questions I've asked here, I'd be very appreciative.

Thanks.

• You already got everything right as far as I can tell. The picture does not specify why its author chose to name the diffraction (or illumination?) angle $2\,\vartheta$. Just as with any diffraction, you can (in theory) expect multiple diffraction orders but note that even observing the first one is sufficiently challenging that not everyone may consider any but the +/- 1st ones worthy of being pointed out! – pyramids Mar 27 '15 at 0:46
• I think it means that it's twice the incident angle. I'm not sure why that would be though. – Bob Dylan Mar 27 '15 at 1:06