Chapter 7 - Quantam Theory of the Atom - Questions and Problems - Page 296: 7.73

$484.5nm$

We know that $\nu=\frac{E}{h}$ $\nu=\frac{4.10\times 10^{-19}}{6.626\times 10^{-34}}=6.187\times 10^{14}s^{-1}$ We also know that $\lambda=\frac{c}{\nu}$ $\lambda=\frac{2.998\times 10^8}{6.187\times 10^{14}}=4.845\times 10^{-7}m=484.5nm$