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Question

A red LED emits light at 0.1Watt uniformly around it. The amplitude of the electric field of the light at a distance 1m from the diode is


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Solution

Given data

  1. The power of the LED is P=0.1watt.
  2. The distance where the electric field has to be measured is L=1m.

Intensity of light

  1. Intensity is the energy emitted per unit area in unit time. For a point light source the intensity is I=P4πr2, where, P is the power of the light and r is the distance where the intensity has to measure.
  2. Intensity of a light source is defined by the form, I=12εoE2×c, where, εois the permittivity of free space, E is the electric field, and c is the velocity of light in free space.

Finding the electric field

As we know, 12εoE2×c=P4πr2

So,

E2=P×24πεo×r2×corE2=0.1×2×9×10912×3×108=614πεo=Coulombconstant=9×109C2/N/m2orE=6=2.45orE=2.45V/m.

Therefore, the electric field of the light at a distance 1m from the diode is 2.45volt/m.


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