Manipulate[Plot [{Sin[n*Pi*d/ll*Sin[x]]^2/(Sin[Pi*d/ll*Sin[x]]^2)},{x,-Pi/10,Pi/10},PlotRange->{0.,ym},AxesLabel->{x,y}],{{d,20,"d"},2,100,Appearance->"Labeled"},
{{ll,2,"lambda"},2,100,Appearance->"Labeled"},
{{n,2,"n"},2,20,Appearance->"Labeled"},
{{ym,5,"Y max"},2,50,Appearance->"Labeled"}]