<< Graphics`;
Lambda = 632.8*10^{-3};
a = 2. ;
d = 20. ;

thfirstmax = Lambda/d

fdiff[th_] = 
        Sin[Pi*Sin[th]*a/Lambda]/(Pi*Sin[th]*a/Lambda);

fgrat[th_, Nsl_] = 
        Sin[Nsl*Pi*Sin[th]*d/Lambda]/(Sin[
              Pi*Sin[th]*d/Lambda]);

Intens[th_, Nsl_] = (fgrat[th, Nsl]*fdiff[th])^2;

Plot[Intens[x, 2], {x, -0.75, 0.75}, PlotDivision -> 5000, 
      PlotRange -> {0, 5}]

Plot[Intens[x, 3], {x, -0.75, 0.75}, PlotDivision -> 5000, 
      PlotRange -> {0, 12}]

Plot[{Intens[x, 2], Intens[x, 3], Intens[x, 4]}, {x, -0.1, 0.1}, 
      PlotDivision -> 5000, PlotRange -> {0, 16}, 
      PlotStyle -> {RGBColor[1, 0, 0], RGBColor[0, 1, 0], 
          RGBColor[0, 0, 1]}]