88.
(a) At r = 40 m, the intensity is
I
=
P
πd
2
/4
π(θr)
4(3.0
× 10
−3
W)
π[(0.17
rad)(40 m)]
83 W/m
.
(b) P
= 4πr
I = 4π(40 m)
(83 W/m
) = 1.7
6
W.