Answer
The diameter is $182~cm$
Work Step by Step
We can use Equation (33-45) to find the critical angle $\theta_c$:
$\theta_c = sin^{-1}~\frac{n_2}{n_1}$
$\theta_c = sin^{-1}~\frac{1.00}{1.33}$
$\theta_c = 48.75^{\circ}$
The maximum incident angle of the light is $48.75^{\circ}$
We can find the radius of the circle:
$\frac{r}{80.0~cm} = tan~48.75^{\circ}$
$r = (80.0~cm)~tan~48.75^{\circ}$
$r = 91.2~cm$
Since the diameter is twice the radius, the diameter is $182~cm$
