Bragg's Law with Refractive Correction

[saybrook1]

Hi guys, the x-ray data booklet gives a modified Bragg's Law that seems to be a combination of Snell's and Braggs. I'll post a picture of what this looks like. I've tried combining the two equations and coming up with their answer but can't get a solid derivation. Any help or a point toward a derivation would be awesome. Thanks!

http://imgur.com/a/CvUGz

 

[Charles Link]

Note: This one has already been answered in the Atomic, Solid State, and Comp. Physics section. For further details, please see the discussion in that section.