SATHEE: Modern Physics 7 Question 32

Modern Physics 7 Question 32

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

(2015 Main)

(a) $2.45 V / m$

(b) $1.73 V / m$

(d) $7.75 V / m$

Show Answer

Solution:

Intensity at a distance $r$ from a point source of power $P$ is given by

$\begin{aligned} I & = \frac{P}{4 π r^{2}} \\ Also, I & = \frac{1}{2} ε_{0} E_{0}^{2} c \end{aligned}$

where, $E_{0}$ is amplitude of electric field and $c$ the speed of light. Eqs. (i) and (ii) we get

$\begin{aligned} E_{0} & = \sqrt{\frac{2 P}{4 π ε_{0} r^{2} c}} \\ = \sqrt{\frac{2 \times 9 \times 10^{9} \times 0.1}{(1)^{2} \times 3 \times 10^{8}}} \\ = \sqrt{6} = 2.45 V / m \end{aligned}$

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Modern Physics 7 Question 32

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

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