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