% --- start of displayed preamble in the book --- vardef refractray(expr s,a,p,na,nb)= save tI,tn,ia,ib,I,J,sib;pair I,J; tI=xpart(p intersectiontimes (s--(s+30cm*dir(a)))); if tI>=0: I=point tI of p;draw s--I;tn=angle(direction tI of p)+90; %drawarrow I--(I+dir(tn)*3cm); % normal %drawarrow I--(I+dir(180+tn)*3cm); ia=tn-angle(s-I); % we have na*sind(ia)=nb*sind(ib) sib=na*sind(ia)/nb;ib=-angle(1+-+sib,sib); J=I+dir(tn+180+ib)*2cm;draw I--J withcolor blue; fi; enddef; % --- end of displayed preamble in the book --- defaultfont:="ptmr8r"; warningcheck:=0; beginfig(1) numeric u;u=3mm;path p; p=((5u,-6u)..(4u,-2u)..(4.5u,0)..(5u,5u)) rotated 90; z0=origin; for a=-40 step 5 until 140:refractray(z0,a,p,1,1.5);endfor; draw p withcolor red; endfig; end;