# Calculate the energy of electromagnetic waves of wavelength $3000\: A^{\circ}.$

$\begin {array} {1 1} (a)\;18.4 \times 10^{-20}Wh & \quad (b)\;1.84 \times 10^{-20}Wh \\ (c)\;1.84 \times 10^{-22}Wh & \quad (d)\;18.4 \times 10^{-22}Wh \end {array}$

Energy, $E = \large\frac{hc}{\lambda}$
$=\large\frac{ 6.63 \times 10^{-34} \times 3 \times 10^8 }{ (3000 \times 10^{-10}) }J$
$= 6.63 \times 10^{-19}J$
$1\: Wh = 1J/s \times 3600\: s = 3600\: J$
So, $E = \large\frac{6.63 \times 10^{-19}}{3600}$$Wh = 1.84 \times 10^{-22} Wh$
Ans : (c)