Frequency of light wave, $v=4 \times 10^{14} Hz$
Wavelength, $\lambda_{m}=5 \times 10^{-7} m$
Wavelength of light wave in air
$\lambda_{\text {air }}=\frac{\text { Velocity of light in air }}{\text { Frequency of light }}$
$=\frac{3 \times 10^{8}}{4 \times 10^{14}}$
$=7.5 \times 10^{-7} m$
$\therefore $ Refractive index of medium is given as
$\mu=\frac{\lambda_{\text {air }}}{\lambda_{\text {medium }}}=\frac{7.5 \times 10^{-7}}{5 \times 10^{-7}}$
$=1.5$