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A lamp emits monochromatic green light uniformly in all directions. The lamp is $3\%$ efficient in converting electrical power to electromagnetic waves and consumes $100\,W$ of power . The amplitude of the electric field associated with the electromagnetic radiation at a distance of $5\,m$ from the lamp will be nearly.......$V/m$
$1.34$
$2.68$
$4.02$
$5.36 $
Solution
Wavelength of monochromatic green light $=5.5 \times 10^{-5} \mathrm{cm}$
Intensity $I\,=\,\frac{\text { Power }}{\text { Area }}$
$ = \frac{{100 \times (3/100)}}{{4\pi {{(5)}^2}}}$
$=\frac{3}{100 \pi} \mathrm{Wm}^{-2}$
Now, half of this intensity ( $\mathrm{I}$ ) belongs to electric field and half of that to magnetic field, therefore,
$\frac{1}{2}=\frac{1}{4} \varepsilon_{0} E_{0}^{2} C$
or $\mathrm{E}_{0}=\sqrt{\frac{2 \mathrm{I}}{\varepsilon_{0} \mathrm{C}}}$
$ = \sqrt {\frac{{2 \times \left( {\frac{3}{{100}}\pi } \right)}}{{\left( {\frac{1}{{4\pi \times 9 \times {{10}^9}}}} \right) \times \left( {3 \times {{10}^8}} \right)}}} $
$=\sqrt{\frac{6}{25} \times 30}=\sqrt{7.2}$
$\therefore \mathrm{E}_{0}=2.68 \mathrm{V} / \mathrm{m}$