Explanation

(b)
$\lambda_{1} = 3000\mathring{\mathrm{A}},\ \lambda_{2} = 6000\mathring{\mathrm{A}}$
$E_{1} = \frac{hc}{\lambda_{1}} = \frac{hc}{3000}$
$E_{2} = \frac{hc}{\lambda_{2}} = \frac{hc}{6000}$
$\frac{E_{1}}{E_{2}} = \frac{\frac{hc}{3000}}{\frac{hc}{6000}} = \frac{hc}{3000} \times \frac{6000}{hc} = \frac{2}{1}$
$E_{1}:E_{2} = 2:1$