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  4. Electric field of plane electromagnetic wave propagating through a non-magnetic medium is given by E =20 cos (2 × 1010 t -200 x ) V / m. The dielectric constant of the medium is equal to: (. Take .μ r =1)

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Q. Electric field of plane electromagnetic wave propagating through a non-magnetic medium is given by $E =20 \cos \left(2 \times 10^{10} \,t -200 \,x \right) \,V / m$. The dielectric constant of the medium is equal to :
$\left(\right.$ Take $\left.\mu_{ r }=1\right)$

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Solution:

Speed of wave $=\frac{2 \times 10^{10}}{200}=10^{8} \,m / s$
Refractive index $=\frac{3 \times 10^{8}}{10^{8}}=3$
Now refractive index $=\sqrt{\varepsilon_{ r } \mu_{ r }}$
$3=\sqrt{\varepsilon_{ r }(1)}$
$\Rightarrow \varepsilon_{ r }=9$