size(8cm,0);
import solids;
import graph3;

//Draw 3D right angle (MA,MB)
void drawrightangle(picture pic=currentpicture,
                    triple M, triple A, triple B,
                    real radius=0,
                    pen p=currentpen,
                    pen fillpen=nullpen,
                    projection P=currentprojection)
{
  p=linejoin(0)+linecap(0)+p;
  if (radius==0) radius=arrowfactor*sqrt(2);
  transform3 T=shift(-M);
  triple OA=radius/sqrt(2)*unit(T*A),
    OB=radius/sqrt(2)*unit(T*B),
    OC=OA+OB;
  path3 tp=OA--OC--OB;
  picture tpic;
  draw(tpic, tp, p=p);
  if (fillpen!=nullpen) draw(tpic, surface(O--tp--cycle), fillpen);
  add(pic,tpic,M);
}

currentprojection=orthographic(10,15,3);
currentlight=adobe;

real r=10, h=6; // r=sphere radius; h=altitude section
triple Op=(0,0,h);

limits((0,0,0),1.1*(r,r,r));
axes3("x","y","z");

real rs=sqrt(r^2-h^2); // section radius
real ch=180*acos(h/r)/pi;
path3 arcD=Arc(O,r,180,0,ch,0,Y,50);

revolution sphereD=revolution(O,arcD,Z);
draw(surface(sphereD), opacity(0.5)+lightblue);
draw(shift(0,0,h)*scale3(rs)*surface(unitcircle3),opacity(0.5));

path3 arcU=Arc(O,r,ch,0,0,0,Y,10);
revolution sphereU=revolution(O,arcU,Z);
draw(surface(sphereU), opacity(0.33)+lightgrey);

// right triangle OO'A
triple A=rotate(100,Z)*(rs,0,h);
dot("$O$",O,NW);
dot("$O'$",Op,W);
dot("$A$",A,N);
draw(A--O--Op--A);
drawrightangle(Op,O,A);

if(!is3D())
  shipout(format="pdf", bbox(Fill(paleyellow)));