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

% create an arrow
\usetikzlibrary{calc, angles, quotes, backgrounds}
\begin{tikzpicture}[
  scale=1,
  background rectangle/.style={fill=gray!20},
  show background rectangle
  ]
  % the wedge
  \coordinate (O) at (0, 0); \coordinate (A) at (10, 0); \coordinate (B) at ($(A)+(0,1)$);
  \node [below right, blue] at (O) {$n_1$}; \node [above right, blue] at (O) {$n_1$};
  \draw [ultra thick, blue] (A) coordinate (Q1) -- (O) coordinate (Q2) -- (B) coordinate (Q3)
    pic ["$\theta$", draw, thick, red, above right, angle radius=80]{angle=Q1--Q2--Q3};
  % the air.
  \node [below left] at ($0.15*(A)+0.7*(B)$) {$n$}; \node at ($(B)-0.8*(A)$) {$n$};
  % the sphere and D.
  \coordinate (D) at ($0.5*(A) + 0.5*(B)$);
  \shade [outer color=brown, inner color=white] (D) circle (0.5) ;
  \draw [dotted, thick] (A) -- +(.5,0); \draw [dotted, thick] (B) -- +(.5,0);
  \draw [magenta, dashed, thick, <->] ($(A)+(.5,0)$) -- ($(B)+(.5,0)$);
  \node [magenta, right] at ($(D)+(.5,0)$) {$D$};
  % the lights.
  \coordinate (I) at ($0.4*(A)$); \coordinate (R1) at ($0.5*(B)$); \coordinate (R2) at ($0.6*(A)$);
  \draw [->, orange, thick] ($(I)+(0,2)$) -- (I);
  \draw [->, orange, thick] (R1) -- ($0.5*(R2)+0.5*(I)+(0,2)$);
  \draw [->, orange, thick] (R2) -- ($(R2)+(0,2)$);
  % OPD.
  \coordinate (D1) at ($(R1)+(2,0)$); \coordinate (D2) at ($0.5*(A)+(2,0)$);
  \draw [magenta, dotted, thick] (R1) -- (D1) -- +(.5,0);
  \draw [magenta, dashed, thick, ->] ($(D1)+(0,1)$) -- (D1);
  \draw [magenta, dashed, thick, ->] ($(D2)-(0,.5)$) -- (D2);
  \node [magenta, below right] at (D1) {$d$};
\end{tikzpicture}