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8. Electromagnetic waves — Physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 SciencePhysics8. Electromagnetic waves1 Mark
MCQ
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 :

(Take $\mu_{{r}}=1$ )

  • ✓
    $9$
  • B
    $2$
  • C
    $\frac{1}{3}$
  • D
    $3$
✓

Answer

Correct option: A.
$9$
a
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 $=\sqrt{\varepsilon_{{r}} \mu_{{r}}}$

$3=\sqrt{\varepsilon_{{r}}(1)}$

$\Rightarrow \varepsilon_{{r}}=9$

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