Focare: Focar obiect

$$ \displaylines { \text{pentru} \ f_1 = x_1, \ x_2 \to \infty \ \text{avem:} \\ - \frac{n_1}{f_1} = \frac{n_2 - n_1}{R} \implies f_1 = \frac{- n_1 R}{n_2 - n_1} } $$