= Solution
For the convex vacuum void, use internal <refractive index> $1$, external <refractive index> $n$, and the original $\alpha>0$ in the immersed <thin lens> formula:
$$
\boxed{f_{\rm void}=\frac{n}{2\alpha(1-n)}=-\frac{n}{2\alpha(n-1)}.}
$$
A concave glass <thin lens> of the same curvature magnitude in vacuum has $\alpha_{\rm concave}=-\alpha$ and therefore
$$
\boxed{f_{\rm concave}=-\frac1{2\alpha(n-1)},\qquad f_{\rm void}=n f_{\rm concave}.}
$$
Both elements diverge, but their physical <focal lengths> differ by a factor $n$ for $n>1$. For example, $n=1.5$ makes the void's <focal length> magnitude $1.5$ times that of the concave glass element. Their reduced <optical powers> $n_{\rm outside}/f$ are equal; physical <focal length> also depends on the surrounding <refractive index>.
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