Mdtryhht

What is the best way to draw 3D spherical mirrors like these on tikz?

This may be a starting point. You are expecting others to type in symbols from your screen shot, and it may be that you're expecting too much.

documentclass[tikz,border=3.14mm]standalone
usepackagetikz-3dplot
usetikzlibraryintersections
begindocument
tdplotsetmaincoords90-30
begintikzpicture[tdplot_main_coords,bullet/.style=circle,fill,inner sep=1pt]
 % parameters
 pgfmathsetmacroRadius2.5
 pgfmathsetmacroAngle120
 % left
 beginscope[shift=(-4,0,0),local bounding box=L]
 draw plot[variable=x,domain=0:360,smooth] (0,Radius*cos(x),Radius*sin(x));
 draw[top color=gray!20,bottom color=gray!30,middle color=white] 
 plot[variable=x,domain=90:-90,smooth] (-0.2,Radius*cos(x),Radius*sin(x))
 -- plot[variable=x,domain=-90:90,smooth] (0,Radius*cos(x),Radius*sin(x))
 --cycle ;
 foreach X in 90,140
 draw plot[variable=x,domain=90:-90,smooth]
 (-0.1*Radius*cos(x),cos(X)*Radius*sin(x),sin(X)*Radius*sin(x));
 draw plot[variable=x,domain=0:60,smooth]
 (-0.1*Radius*cos(x),cos(Angle)*Radius*sin(x),sin(Angle)*Radius*sin(x))
 node[bullet] (P1);
 path (-0.1*Radius*cos(-40),cos(Angle)*Radius*sin(-40),sin(Angle)*Radius*sin(-40))
 node[bullet] (P0);
 draw (0,0,-1.02*Radius) -- (0,0,-1.2*Radius) coordinate (l);
 endscope 
 % left
 beginscope[shift=(4,0,0),local bounding box=R]
 foreach X in 90,140
 draw[dashed] plot[variable=x,domain=90:-90,smooth]
 (-0.2+0.1*Radius*cos(x),cos(X)*Radius*sin(x),sin(X)*Radius*sin(x));
 draw[dashed] plot[variable=x,domain=90:270,smooth]
 (-0.2,Radius*cos(x),Radius*sin(x));
 draw[top color=gray!20,bottom color=gray!30,middle color=white] 
 plot[variable=x,domain=90:-90,smooth] (-0.2,Radius*cos(x),Radius*sin(x))
 -- plot[variable=x,domain=-90:90,smooth] (0,Radius*cos(x),Radius*sin(x))
 --cycle ;
 path[name path=left boundary] plot[variable=x,domain=90:-90,smooth]
 (-0.2,Radius*cos(x),Radius*sin(x));
 path (-0.2+0.1*Radius*cos(30),0,Radius*sin(30)) 
 node[bullet,label=above:$P_2$] (P2)
 (-0.2+0.1*Radius*cos(-30),0,Radius*sin(-30)) node[bullet] (P3);
 path[name path=ray 1] (P1) -- (P2) coordinate[pos=0.5] (A);
 path[name path=ray 2] (P2) -- (P0);
 path[name path=ray 3] (P0) -- (P3);
 path[name path=ray 4] (P3) -- (P1);
 path[name intersections=of=left boundary and ray 1,by=i1,
 name intersections=of=left boundary and ray 2,by=i2,
 name intersections=of=left boundary and ray 3,by=i3,
 name intersections=of=left boundary and ray 4,by=i4];
 draw[thick,-stealth] (i2) -- (P0) -- (i3) (P1) -- (i1) (P1) -- (A);
 draw[thick,-stealth,shorten >=1.5cm] (i4) -- (P1);
 draw[dashed] (i1) -- (P2) -- (i2) (i3) -- (P3) -- (i4);
 draw plot[variable=x,domain=0:360,smooth] (0,Radius*cos(x),Radius*sin(x));
 draw (0,0,-1.02*Radius) -- (0,0,-1.2*Radius) coordinate (r);
 endscope 
 draw[stealth-stealth] (l) -- (r) node[midway,fill=white]$r+varepsilon$;
endtikzpicture 
enddocument