1. The problem statement, all variables and given/known data When x-rays are diffracted off NaCl, constructive interference is observed at an angle of 16.3 degrees. Determine the lattice constant of NaCl. Frequency of the X-ray is 1.948 x 10^18 hz Theta = 16.3 degrees 2. Relevant equations (As far as what we're told to use and what's in the book) 2d * Sin (Theta) = m * Lambda 3. The attempt at a solution After looking through the problem it is very clear that I am unable to find the "lattice constant" from the information given in the book. I've found the following equations: 1) d = a/sqrt(h2+k2+l2) 2) Lambda/2a= sin 2 Theta/h2+k2+l2 I've tried deriving #2 down to where it is usable and I get: a2 = (Lambda2/4) * (h2+k2+l2/Sin2 Theta) I've been unable to figure out what the miller indices are (h k and l), but everything I've tried doesn't get anywhere near the values expected online of 5.63-5.65. Am I going about this completely wrong or am I just overthinking the problem? Any help would be much appreciated.