Cylindrical Wavefronts: Linear Source Intensity at 60m

[denian]

for a linear source in three dimensions, the wavefront are cylindrical.

and intensity, I is inversely proportional to r ( distance from the source )

is the fact given correct??


here's the question.
cylindrical wavefronts are emitted by a linear source of length 4.0 m, at a rate of 50 microwatt.
what is the intensity of this wave at a distance of 60m from the source? i just want to check out whether the answers in the book is correct or not..

thank you.

 

[Hurkyl]

Only when the source is approximately infinitely long (that really does make sense...) does it generate cylindrical wavefronts.

Since the distance from the wire is much greater than the length of the wire, my guess is that the problem is best approximated by spherical wavefronts, so intensity decays as 1/r^2

You can, of course, always whip out the integral and compute it without the approximations.

 

[denian]

thanks.
so how bout my 2nd question?