Caustics from waves

\[Theta] = Pi/3;
f[x_, y_] :=
 0.8 Sin[xt] Cos[yt] /. {xt -> x Cos[\[Theta]] - y Sin[\[Theta]],
   yt -> y Cos[\[Theta]] + x Sin[\[Theta]]};
g = Derivative[1, 0][f];
n = 70; r = 0.85;
Manipulate[

 Show[
  Graphics[{
    FaceForm[RGBColor[0.95, 0.95, 1.0]],
    EdgeForm[
     Directive[Thickness[0.01],
      RGBColor[0.6365758754863813, 0.8532539864194705,
       0.970244907301442]]],
    Polygon[
     Join[{{-1, 5}},
      Table[{x, f[x, y]}, {x, 0, 4 Pi, (4 Pi)/50}], {{4 Pi + 1,
        5}}]],
    Table[
     Block[{p1, p2, p3, m, sin1, sin2, cos2, d\[Theta], xt},
      p1 = {x, 4}; p2 = {x, f[x, y]}; m = g[x, y]; sin1 = m;
      sin2 = r sin1; d\[Theta] = ArcSin[sin1] - ArcSin[sin2];
      cos2 = Sqrt[1 - sin1^2];
      p3 = p2 + (p2[[2]] + 3) {Sin[d\[Theta]]/Cos[d\[Theta]], -1};
      xt = p3[[1]];
      {RGBColor[0.781048294804303, 0.6390783550774395,
        0.42232394903486686], Line[{p1, p2}], White, Line[{p2, p3}],
       Opacity[0.2], FaceForm[White], EdgeForm[],
       Rectangle[{xt - 2 (4 Pi/n), -3.5}, {xt + 2 (4 Pi/n), -3}]}
      ]
     , {x, 0, 4 Pi, (4 Pi)/n}]},
   Background ->
    RGBColor[0.10379186694132907, 0.2430914778362707,
     0.49544518196383613], PlotRange -> {{0.5, 4 Pi - 0.5}, {-3.5, 4}}]

  ]
 , {y, 0, 4 Pi}]