Dioptrul plan
$$
\displaylines {
\begin{equation}
\left.
\begin{aligned}
& R \to \infty \\
& \frac{n_2}{x_2} - \frac{n_1}{x_1} = \frac{n_2 - n_1}{R}
\end{aligned}
\right\vert
\implies
\begin{aligned}
\frac{n_2}{x_2} = \frac{n_1}{x_1} \implies
\end{aligned}
\end{equation}
\\
\implies n_1 x_2 = n_2 x_1 \implies \beta = 1\\
\\
f_1 = - \infty \\
f_2 = \infty
}
$$