A lamp emits monochromatic green light uniformly in all directions. The lamp is 3 % efficient in converting electrical power to electromagnetic waves…
JEE Main 2014 — Physics Electromagnetism
2014mcqhard
A lamp emits monochromatic green light uniformly in all directions. The lamp is 3% efficient in converting electrical power to electromagnetic waves and consumes 100W of power. The amplitude of the electric field associated with the electromagnetic radiation at a distance of 5m from the lamp will be nearly:
Official previous-year question
Held on 12 Apr 2014 · Verified 6 Jul 2026.
Options
A
1.34V/m
B
2.68V/m
C
4.02V/m
D
5.36V/m
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Solution
Wavelength of monochromatic green light =5.5×10−5cm Intensity I = Area Power =4π(5)2100×(3/100)=100π3Wm−2 Now, half of this intensity (I) belongs to electric field and half of that to magnetic field, therefore, 2I=41ε0E02C or E0=ε0C2I=(4π×9×1091)×(3×1082×(1003π)=256×30=7.2∴E0=2.68V/m
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