I Understanding Diffraction Condition in Kittle's Intro to Solid State Physics

Mart1234
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Diffraction Condition derivation in Kittle's Introduction to Solid State Physics
I am going over the diffraction condition section in Kittle's Introduction to Solid State Physics physics and I am having a hard time understanding why the phase difference angle for the incident wave is positive while the phase angle difference for the diffracted wave is negative. Thank you for the help.

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Mart1234 said:
TL;DR Summary: Diffraction Condition derivation in Kittle's Introduction to Solid State Physics

I am having a hard time understanding why the phase difference angle for the incident wave is positive while the phase angle difference for the diffracted wave is negativeiy
I see in the figure attached that
\mathbf{k}\cdot \mathbf{r}>0.....(1)
The phase of upside or outside wave advances until reflection.
\mathbf{k'}\cdot \mathbf{r}<0.....(2)
The phase of upside or outside wave advance after reflection also so sign must be inverted for addition to (1) to give the full phase difference by reflection.
 
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