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

In summary, the diffraction condition derivation in Kittle's Introduction to Solid State Physics involves understanding why the phase difference angle for the incident wave is positive while the phase angle difference for the diffracted wave is negative. This is due to the fact that the phase of the incident wave advances until reflection, while the phase of the diffracted wave advances after reflection, resulting in an inverted sign when added to the original positive phase difference.