## Physics: Principles with Applications (7th Edition)

$61.0^{\circ}$; The light must start in the crown glass.
Total internal reflection happens when the light in the material with a higher index is incident upon an interface. The light must start in the crown glass. At the critical angle, the refracted angle is $90^{\circ}$. Find the critical angle of incidence using Snell’s law. Here “1” designates crown glass, and “2” designates water. $$n_1sin\theta_1= n_2sin\theta_2$$ $$\theta_1=sin^{-1}(\frac{n_2}{n_1}sin90^{\circ})$$ $$=sin^{-1}(\frac{1.33}{1.52}sin90^{\circ}) =61.0^{\circ}$$