X-Ray Diffraction Equation (Mineralogy HW)

[Maxilen]

Homework Statement

Okay so, I was hoping someone out there might be able to assist me with a Mineralogy problem I have. The concept is based around X-Ray Diffraction.

The question:

Derive a diffraction equation from the illustration below that relates the reflection angle (theta) to the wavelength (lambda) and the d spacing (d). Show all of your work so that it is clear that you understand the physics and the math in this important derivation.

Here is a picture of the diagram given...sorry for the poor quality :)
http://yfrog.com/5m0331102025j

I've been banging my head against this for two days now, and I just don't seem to be getting anywhere with this...Ive tried google, my book, everything at my disposal, I just can't seem to figure this out at all...I am at a stand still because there are 5 other questions related to this problem, but i need to solve this first...can anyone help...Thanks in advance for your time! :)

 

[Borek]

I guess it is not for any theta, but for a specific value of theta? One that gives some observable effect? For that effect to happen - is there anything you can tell about FG?

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[PhaseShifter]

Start with one of the fundamental considerations:
in terms of that diagram, what are the conditions for constructive vs. destructive interference of the reflected waves?