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# What is the value of frequency at which an e.m wave must be propagated for the D- region to have a refractive index of $0.49$ Electron density for D- layer is $10^9m^{-3}$

$\begin{array}{1 1} 3.27 \times 10^5\;Hz \\327 \times 10^5\;Hz \\ 32.7 \times 10^5\;Hz \\ 0.327 \times 10^5\;Hz\end{array}$

given $\mu = 0.49$
$N= 10^9\;m^{-3}$
$\mu = \sqrt{ 1- \Large\frac{81.45\;N}{\alpha^2}}$
$0.49 = \sqrt{1- \Large\frac{(81.45 \times 10^9)}{\alpha^2}}$
or $\alpha= \bigg(\large\frac{81.45 \times 10^9}{1-0.24} \bigg)^{1/2}$
$\qquad= 3.27 \times 10^5 \;Hz$