a b c d

d

$ m = \frac{1}{n} = -\frac{v}{-u}$ $ v = \frac{u}{n}$ Mirror Equation  $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$ $ \frac{n}{u} + \frac{1}{-u} = \frac{1}{f}$ $\Rightarrow u = (n - 1)f$