Answer
$0.021\;m$
Work Step by Step
In the previous section of this problem, we have calculated that the frequency of the radiation is $v=1.40\times 10^{10}\;Hz$
Let, $\;\lambda$ be the wavelength of the radiation that would induce a transition between the two states. Now we can write
$\lambda=\frac{c}{v}$
or, $\lambda=\frac{3\times 10^{8}}{1.40\times 10^{10}}\;Hz$
or, $\lambda\approx0.021\;m$
Therefore, the wavelength of the radiation is $0.021\;m$
