Q147 / 180 · AIIMS 2003 · Physics

In short wave communication, waves of which of the following frequencies will be reflected back by the ionospheric layer having electron density $10^{11}$ per $\mathrm{m}^3$ ?

  • Correct answer
    A
    $2 \mathrm{MHz}$
  • B
    $10 \mathrm{MHz}$
  • C
    $12 \mathrm{MHz}$
  • D
    $18 \mathrm{MHz}$.

Solution

We know that the artificial frequency of a sky wave for reflection from an ionospheric layer of the atmosphere is given by, $v_c=9 N^{1 / 2}$ where $n$ is the number density of electrons $/ \mathrm{m}^3$. It is given that $N=10^{11} / \mathrm{m}^3$ $$ \therefore \quad v_c=9 \times\left(10^{11}\right)^{1 / 2}=2.8 \times 10^6 \mathrm{~Hz}=2.8 \mathrm{MHz} \text {. } $$ Hence the wave of frequency $2 \mathrm{MHz}$ will be reflected back.

Hence, the correct answer is option: 'A'.

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