93. The source being isotropic means Asphere = 4πr2 is used in the intensity definition I = P/A. Since intensity is proportional to the square of the amplitude (see Eq. 18-27), this further implies I2 = I1
sm2 sm1
2 =
P/4πr22 = P/4πr12
r1 r2
2
or sm2 /sm1 = r1 /r2 . (a) With I = P/4πr2 = (10 W)/4π(3.0 m)2 = 0.088 W/m2 . (b) Using the notation A instead of sm for the amplitude, we find 3.0 m A4 = A3 4.0 m
=⇒ A4 =
3 A3 . 4