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The intensity of a light pulse travellin...

The intensity of a light pulse travelling along a communication channel decreases exponetially with distance x according to the relation `I=I_(0)e^(-ax)` where `I_(0)` is the intensity at x=0 and `alpha` is the attenuation constant. The percentage decrease in intensity after a distance of `(("In"4)/(alpha))` is

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(a) Given, the intensity of a light pulse `I=I_(0)e^(-alphax)`,
where, `I_(0)` is the intensity at x = 0 and `alpha` is constant.
According to the question, I = 25% of `I_(0)=(25)/(100).I_(0)=(I_(0))/(4)`
Using the formula mentioned in the question,
`I=I_(0)e^(-alphax)`
`(I_(0))/(4)=I_(0)e^(-alphax)`
or `(1)/(4) = e^(-alphax)`
Taking log on both sides, we get
`ln1-ln4=-alphaxlne" "(becauselne=1)`
`-ln4=-alphax`
`x=(ln4)/(alpha)`
Therefore, at distance `x=(ln4)/(alpha)`, the intensity is reduced to 75% of initial intensity. (b) Let `alpha` be the attenuation in dB/km. If x is the distance travelled by signal, then
`10log_(10)((I)/(I_(0)))=-alphax " ....(i)"`
where, `I_(0)` is the intensity initially.
According to the question, I = 50 % of `I_(0)=(I_(0))/(2)` and x = 50 km
Putting the value of x in Eq. (i), we get
`10log_(10).(I_(0))/(2I_(0))=-alphaxx50`
`10[log1-log2]=-50 alpha`
`(10xx0.3010)/(50)=alpha`
`therefore` The attenuation for an optical fibre
`alpha = 0.0602` dB/km
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