Answer
$1.3*10^{-4}m$
Work Step by Step
We are given that the angular diameter of the ring is 1.35 times the angular diameter of the Moon, which is$ 0.50^{\circ}$, so we obtain an angular diameter of:
$\theta=1.35*0.50^{\circ}$
$\theta=0.675^{\circ}$
$\theta=0.0117rad$
Using the largest visible wavelength of light, $\lambda = 700 *10^{-9}m$, in equation (36-14); we can find the diameter of the water drops.
$\theta_R= \frac{1.22\lambda}{d}$
$d= \frac{1.22\lambda}{\theta_R}$
$d\approx \frac{1.22*700 *10^{-9}m}{0.0117rad}$
$d\approx 1.3*10^{-4}m$
