<< 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]}]

Lambda2 = 633.6*10^{-3}

fdiff2[th_] = 
        Sin[Pi*Sin[th]*a/Lambda2]/(Pi*Sin[th]*a/Lambda2);

fgrat2[th_, Nsl_] = 
        Sin[Nsl*Pi*Sin[th]*d/Lambda2]/(Sin[
              Pi*Sin[th]*d/Lambda2]);

Intens2[th_, Nsl_] = (fgrat2[th, Nsl]*fdiff2[th])^2;

Plot[{Intens[x, 200], Intens2[x, 200]},  {x, 0.031,  .032},  
      PlotDivision  ->  5000,  PlotRange  ->  {0, 100000},  
      PlotStyle -> {RGBColor[1, 0, 0],  RGBColor[0, 1, 0]}]

\(So\ here\ we\ see\ that\ the\ plots\ have\ different\ peaks\ at\ 200. \ \ I\ focused\ the\ peak\ around\ the\ first\ max\ as\ defined\ by\ prof . \ 
        nelson\ above . \ \ This\ section\ was\ graded\ generously\ since\ most\ people\ had\ trouble\ with\ it . \ \ They\ way\ you\ define\ "distinct peaks"\ also\ has\ a\ great\ effect\ on\ your\ \(\(answer\)\(.\)\)\)

Plot[{Intens[x, 2000], Intens2[x, 2000]},  {x, 0.0316,  .03175},  
      PlotDivision  ->  5000,  PlotRange  ->  {0, 4500000},  
      PlotStyle -> {RGBColor[1, 0, 0],  RGBColor[0, 1, 0]}]

Above shows very distinct peaks.