Answer
$E = 5.9\times 10^{-6}~eV$
Work Step by Step
We can find the photon energy:
$E = \frac{hc}{\lambda}$
$E = \frac{(6.63\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{0.21~m}$
$E = 9.47\times 10^{-25}~J$
$E = (9.47\times 10^{-25}~J)(\frac{1~eV}{1.6\times 10^{-19}~J})$
$E = 5.9\times 10^{-6}~eV$
