Question

A red LED emits light of 0.1 watt uniformaly around it. The amplitude of the electric field of the light at a distance of 1m from the diode is
A. `1.73 Vm^(-1)`
B. `2.45Vm^(-1)`
C. `5.48Vm^(-1)`
D. `7.75Vm^(-1)`

Answer 1

Correct Answer - B
Consider the LED as a point source of light.
Let power of the LED is P.

Intensity at r from the source
`I=(P)/(4pir^(2)) " " ` …(i)
As wer know that `I=(1)/(2)epsilon_(0) E_(0)^(2)C`
`or E_(0)^(2)=(2P)/(4pi epsilon_(0) r^(2)c)=(2xx0.1xx9xx10^(9))/(1xx3xx10^(8))`
`or E_(0)^(2) =6 rArr E_(0) = sqrt(6) =2.45 Vm^(-1)`