Answer
$3.28\times 10^{15}Hz$
Work Step by Step
We can find the required minimum frequency as follows:
$|\Delta E|=|\frac{-13.6}{1^2}|$
$|\Delta E|=(13.6)(1.6\times 10^{-19}J)$
We know that
$E=hf$
$\implies hf=(13.6)(1.6\times 10^{-19}J)$
This can be rearranged as:
$f=\frac{(13.6)(1.6\times 10^{-19})}{6.63\times 10^{-34}}$
This simplifies to:
$f=3.28\times 10^{15}Hz$