mjhjmtu

Please consider this MWE:

documentclassarticle

usepackagetikz
usetikzlibraryshapes.geometric

begindocument
begintikzpicture
 node (C) [cylinder,rotate=180,draw,minimum height=4cm,minimum width=1.5cm] ;
 draw [thick,->] ([xshift=-20pt]C.top) -- ([xshift=-80pt]C.bottom) node ;
 draw [thick] ([xshift=-80pt]C.bottom) -- ([xshift=-30pt]C.bottom) node ;
endtikzpicture
enddocument

With your help, I want to draw:

Thanks!

This is so simple that even marmots can do it. (EDIT: compute the scattering angle.)

documentclassarticle

usepackagetikz
usetikzlibraryshapes.geometric,intersections,decorations.markings,angles,calc

begindocument
begintikzpicture[thick,font=sffamily]
 node[ellipse,draw,inner sep=0pt,minimum height=1.5cm,minimum width=4mm]
 (L);
 draw[name path=tube] (L.north) -- ++ (3,0) coordinate (A) node[above] A
 -- ++ (-40:2) coordinate (B) node[right] B
 -- ++ (-80:3) 
 [rotate=10]arc(0:-180:0.75 and 0.2)[rotate=0] -- ++(110:2) to[out=90,in=0] ++ (-0.5,0.5) 
 --(L.south);
 draw[dashed] (A) to[out=0,in=110,looseness=1.3] (B);
 path[name path=test ray] (L.center) -- ++ (4.5,0);
 draw[ultra thick,-latex,name intersections=of=tube and test ray,
 postaction=decorate,decoration=markings,
 mark=at position 2.5cm with draw[-latex] (-5pt,0) -- (0,0);] 
 let p1=($(intersection-1)-(A)$),n1=2*atan2(y1,x1) in
 ([xshift=-4mm]L.center) -- (intersection-1) -- ++ (n1:2.5);
 draw[dashed,very thick,blue] (intersection-1) -- ($(intersection-1)!3!90:(B) $) 
 coordinate (x3);
 draw[dashed,very thick,red] (intersection-1) -- ($(intersection-1)!0.5cm!(B)$)
 coordinate (x4);
 draw pic[draw=purple,angle radius=5mm] angle=x4--intersection-1--x3; 
 draw pic[draw=black,angle radius=5mm,pic text=$theta$,
 angle eccentricity=1.3] angle=A--intersection-1--L; 
endtikzpicture
enddocument