A red L E D emits light at 0 . 1 watt uniformly around it. The amplitude of the electric field of the light at a distance of 1 m from the diode is:

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For a point source of power = P, then intensity at a point at a separation x from the source is, $I = \frac{P o w e r}{A r e a} = \frac{P}{4 π x^{2}}$ $∵$ The average intensity of E.M. wave is given by, $I = \frac{1}{2} c ε_{0} E_{o}^{2}$ $\Rightarrow E_{0} = \sqrt{\frac{2 P}{4 π ε_{0} c x^{2}}}$

$∵ \frac{1}{4 π ε_{0}} = 9 \times 10^{9}, P = 0.1 W, x = 1 m$ , $c = 3 \times 10^{8} m s^{- 1}$

$\Rightarrow E_{0} = \sqrt{\frac{2 \times 0.1 \times 9 \times 10^{9}}{3 \times 10^{8} \times 1^{2}}} = \sqrt{6} = 2.45 V m^{- 1}$

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