Explanation

(a)
$E = 3 \times 10^{- 12}ergs$
$\lambda = ?$
$h = 6.62 \times 10^{- 27}ergs$
$c = 3 \times 10^{10}cms^{- 1}$
$E = \frac{hc}{\lambda}$
$3 \times 10^{- 2} = \frac{6.62 \times 10^{- 27} \times 3 \times 10^{10}}{\lambda}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \lambda = \frac{6.62 \times 10^{- 27} \times 3 \times 10^{10}}{3 \times 10^{- 12}}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 6.62 \times 10^{- 5}cm$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 662 \times 10^{- 7}cm$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 662 \times 10^{- 9}m$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 662\ nm.$