Answer
$f_{min}=4.3\times 10^{14}Hz$
Work Step by Step
As we know that
$f=\frac{c}{\lambda}$
Thus minimum frequency is
$f_{min}=\frac{c}{\lambda_{max}}$
We plug in the known values to obtain:
$f_{min}=\frac{3\times 10^8}{700\times 10^{-9}}=4.3\times 10^{14}Hz$
