\documentclass[tikz]{standalone}
\usepackage{tikz}

% create an arrow
\usetikzlibrary{calc, snakes, backgrounds}
\begin{tikzpicture}[
  scale=1,
  background rectangle/.style={fill=gray!10},
  show background rectangle
  ]
  % the convex len
  \coordinate (O) at (0,0);
  \draw [thick] (O) arc (180+60:360-60:5)
    coordinate [pos=0.5] (B) coordinate [pos=1] (A) coordinate [pos=0.9] (D);
  \node [below right] at (D) {$d$};
  % the plane len
  \draw [thick] (B -| O) -- (B -| A);
  % doing math
  \coordinate (OC) at ($(B)+(0, 5)$); \coordinate (OD) at ($(B)!(D)!(OC)$);
  \draw (OC) -- (B); \draw (D) -- (D |- B); \draw (D) -- (OD);
  \draw (D) -- (OC) node [pos=.5, above right] {$R$};
  \draw [decoration={brace, mirror, raise=.1cm}, decorate] (B) -- (D |- B)
    node [pos=0.5, below, yshift=-.2cm] {$r$};
  \draw [decoration={brace, mirror, raise=.05cm}, decorate] (OC) -- (OD)
    node [pos=0.5, left, xshift=-.1cm] {$R-d$};
\end{tikzpicture}
