\documentclass{standalone}

\usepackage{tikz}
\begin{document}

\begin{tikzpicture}

\draw [ultra thin,cyan,->] (-5,0)--(5,0);

\draw [ultra thin,cyan,->] (0,-.5)--(0,3);
\draw [ultra thin,cyan,dashed] (2,0)--(2,2.5);
\draw [ultra thin,cyan,dashed] (4,0)--(4,1);
\draw [ultra thin,cyan,dashed] (-2,0)--(-2,2.5);
\draw [ultra thin,cyan,dashed] (-4,0)--(-4,1);

\node at (-4, -.2) {$2l$};
\node at (-2, -.2) {$-l$};
 \node at (0, -.2) {$0$};
\node at (2, -.2) {$l$};
\node at (4, -.2) {$2l$};
  
  
\draw [thick] (-5,1.75)--(-4,1)--(-2,2.5)--(0,1)--(2,2.5)--(4,1)--(5,1.75);
\end{tikzpicture}
\end{document}
