reflection

\documentclass[]{standalone}

\usepackage[]{graphicx}

\usepackage{amsmath}
\usepackage{bm}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\tikzset{>=latex}
\usetikzlibrary{backgrounds,fit,shadows,patterns,decorations.markings,quotes,angles,arrows.meta}
\usepackage{steinmetz}
\newcommand*{\equal}{=}

\usepackage{tikz-3dplot}
\usetikzlibrary{decorations.markings}

\tikzset{->-/.style={decoration={
  markings,
  mark=at position #1 with {\arrow{>}}},postaction={decorate}}}

\begin{document}
\begin{tikzpicture}[scale=2.5]
   \draw[-,thick] (-0.7,-0.7) to (0.7,0.7);
   \draw[] (0.6,0.6) node[below right]{\footnotesize $f(x,y)=0$};
   \draw[->, very thick] (-1, 0) coordinate(A) node[left]{$\bm{i}$} to (0,0) coordinate(O) node[below right]{$\bar{\bm{x}}$};
   \draw[->, very thick] (0, 0) to (0,1) coordinate(B) node[above]{$\bm{o}$} ;   
   \draw[->, very thick] (0, 0) to (-0.8,0.8) coordinate(N) node[above]{$\bm{n}$} ; 
   %\draw[->, very thick, red] (-1/2,-1/2) to node[midway, below]{$\bm{p}_t$} (0,0);
   \draw[->, very thick, red] (0,0) to node[near end, below]{$\bm{p}_n$} (-0.5,0.5);                  
   \draw[->, very thick, red] (-1,0) to node[midway, above]{$\bm{p}_t$} (-0.5,0.5);            
   \draw[->, very thick, red] (-0.5,0.5) to node[midway, above]{$\bm{p}_t$} (0,1);
   \pic [draw, <->,
      angle radius = 7mm,
      angle eccentricity=1.3, 
      "$\theta$"] {angle = N--O--A};
   \pic [draw, <->,
      angle radius = 7mm,
      angle eccentricity=1.3, 
      "$\theta$"] {angle = B--O--N};
%  \end{axis}
\end{tikzpicture} 

\end{document}