Request explanation of use of [[1, 2]] [closed]

Question

The following is the code for Kronig-Penney model from the book A Physicist's Guide to Mathematica by Patrick T. Tam. I would be grateful if someone could explain the code to me, especially line with FindRoot. What is the role of [[1, 2]] there?

kmPlot[l_?Positive, nb_Integer?Positive, mp_Integer /; mp > 10] :=
  Module[{K, Q, d, r},
   K[n_, 1] := n*Pi;
   Q[n_, m_] := n*Pi - (m - 1)*Pi/(mp - 1);
   K[n_, m_] :=  
     K[n, m] = 
       FindRoot[Cos[Q[n, m]] == Cos[x] + (l/2)*(Sin[x]/x), {x,K[n, m - 1]}][[1, 2]];
   d[n_, m_] := K[n, m]^2;
   r[n_] := r[n] = Table[{Q[n, m], d[n, m]}, {m, 1, mp}];
   Graphics[
     Flatten[Table[{Line[r[n]], Line[{-#[[1]], #[[2]]} & /@ r[n]]}, {n,1, nb}]], 
     Ticks ->  {Table[{I*Pi, I "p"}, {I, -nb, nb}], Automatic}, 
     Axes -> True, AxesLabel -> {"q(1/a)", "E"}, 
     AspectRatio -> 1/GoldenRatio]];

Answer 1

It is easy, given Mathematica's highly interactive nature, to answer questions like this yourself by making some simple experiments.

First it is alway good to look at strange notation with FullForm, which will shows you the internal form. Hold is used here to suppress evaluation of the form. This tells you that you should consult Part in the documentation.

 Hold[x[[1, 2]]] // FullForm

Hold[Part[x, 1, 2]]

Next compare what FindRoot produces with and without [[1, 2]].

 FindRoot[Sin[x] == x/2, {x, 2}]

{x -> 1.89549}

 FindRoot[Sin[x] == x/2, {x, 2}][[1, 2]]

1.89549

So you see that [[1, 2]] is used to extract the value of the root from the expression returned by FindRoot.