Answer
a) $\implies \theta_w=48.8^{\circ}$
b) the answer to part (a) does not depend on the thickness of the film.
Work Step by Step
(a) We can find the angle of refraction in water as follows:
$\theta_w=sin^{-1}(\frac{n_a}{n_w}sin\theta_a)$
We plug in the known values to obtain:
$\theta_w=sin^{-1}(\frac{1.00}{1.33})$ (as $\theta_a=90^{\circ}$)
$\implies \theta_w=48.8^{\circ}$
(b) We know that the answer to part (a) does not depend on the thickness of the film. It depends on the refractive index of water and the air and the angle of incidence.
