Calculate the wavelength,frequency and wave number of a light wave whose period is $2.0 \times 10^{-10} \,s$.

Frequency $(\nu)$ of light $= \frac{1}{\text{Period}}$
$= \frac{1}{2.0 \times 10^{-10} \,s} = 5.0 \times 10^{9} \,Hz$
Wavelength $(\lambda)$ of light $= \frac{c}{\nu}$
Where $c$ (velocity of light in vacuum) $= 3 \times 10^{8} \,m/s$
$\lambda = \frac{3 \times 10^{8} \,m/s}{5.0 \times 10^{9} \,s^{-1}} = 0.06 \,m = 6.0 \times 10^{-2} \,m$
Wave number $(\bar{\nu}) = \frac{1}{\lambda} = \frac{1}{6.0 \times 10^{-2} \,m} = 16.66 \,m^{-1}$