Code for http://math.stackexchange.com/a/1634593/290189

mse1634593.tex

\documentclass[tikz]{standalone}
\usetikzlibrary{calc,through}
\begin{document}
\begin{tikzpicture}[scale = 2]
  \coordinate [label=left:$O$] (O) at (0,0);
  \coordinate [label=left:$L$] (L) at (0.7,1.1);
  \coordinate [label=below:$S_n$] (Sn) at (0.6,0.7);
  \coordinate (SqCorn1) at (0.2,0.6);
  \coordinate (SqCorn2) at (1.2,1.6);
  \coordinate (SqCorn3) at (1.2,0.6);
  \coordinate (D8h) at (0.7,1.6);

  \draw [fill] (O) circle [radius = 0.025];
  \draw (O) circle (1);
  \draw (O) -- (120:1) node[pos=0.5,above] {$r = 1$};
  \node [below] at (0,-1) {$D$};

  \draw [blue,fill] (L) circle [radius = 0.025];
  \draw [blue,dashed] (SqCorn1) rectangle (SqCorn2);
  \draw [<->,blue] (L) -- (D8h) node[pos=0.5,right]
  {$\frac{\varepsilon}{\sqrt{2}}$};
  \draw [<->,blue] (L) -- (SqCorn3) node[pos=0.5,above] {$r = \varepsilon$};
  \node (E) [draw,blue,dashdotted,circle through=(SqCorn1),label=right:$C$]
  at (L) {};

  \draw (0,0) -- (L);
  \draw [fill] (Sn) circle [radius = 0.025];
\end{tikzpicture}
\end{document}