Answer
$5.36\;cm$
Work Step by Step
The energy required of a photon to induce a transition of an electron spin from parallel to antiparallel orientation in a magnetic field is given by
$hf=2\mu_zB$
or, $\frac{hc}{\lambda}=2\mu_zB$
or, $\lambda=\frac{hc}{2\mu_zB}$
For electron, the spin magnetic moment
$\mu_z=1\;\mu_B=9.27\times 10^{-24}\;J/T$
Substituting the gives values, we get,
$\lambda=\frac{6.63\times 10^{-34}\times 3\times 10^{8}}{2\times9.27\times 10^{-24}\times 0.2}\;m$
or, $\lambda=0.0536\;m$
or, $\lambda=5.36\;cm$
$\therefore\;$ The wavelength associated with a photon is $5.36\;cm$
