What is the logic behind the peaks?
I understand (01),(10),(11) represent h,k from 1c.
However, when I want to find theta, I need d_{h,k} and lambda?
the key point here is that the scattering angle is proportional to the length of the reciprocal lattice vector |\mathbf{G}| = |h\mathbf{b}_1+k\mathbf{b}_2| (see the X-ray lecture). In this particular case, the vector with (h,k) = (1,0) is shorter than the vector with (h,k)=(0,1), hence the 1,0 peak appears at a smaller angle than the 0,1 peak