I'm a newbie trying to learn from the book "Photonic Crystals" 2nd ed. 
(a True Masterpiece!).  I have a simple question about how to interpret 
isofrequency diagrams like Fig. 14 & 15 of Chapter 10 (p. 222-225).  

I expect that I can get the ordinary (reflective) Bragg condition:

2d cos(theta) = n lambda, [d = interplane distance = sqrt(2) * a etc]

that is, if I fix the contour (omega = constant) and scan the incident angle 
(i.e., the dashed line in Fig. 14 Right), there should be a specific angle 
which satisfies the above condition and therefore gives a strong reflective 
peak (the opal color).  

I understand that the selected contour value in Fig. 14R (0.276) may be 
too low for the Bragg condition (we need a higher band), but nevertheless, 
I guess there must be a way to see such a general phenomenon via 
something similar to Fig. 14 (and the related discussion in the text).  

But I cannot see how to get this condition by looking at the isofrequency 
diagram or band contour plot.  Could someone help me out?  I'd appreciate 
it very much.  


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