## Chemistry: Principles and Practice (3rd Edition)

$1.4\times10^{7}s^{-1}$
λ = 21 m c = $2.997925\times10^{8} ms^{-1}$ ν = c/λ = $\frac{2.997925\times10^{8} ms^{-1}}{21 m}$ =$1.4\times10^{7} s^{-1}$