The following discussion of Snell's Law of Refraction* (named after Willebrord Snell, 1580-1626) is needed for Light, sound, and other waves travel at different speeds, depending on the media (air, water, wood, and so on) through which they pass. Suppose that light travels from a point $A$ in one medium, where its speed is $v_1$, to a point $B$ in another medium, where its speed is $v_2$. Refer to the figure, where the angle $\theta_1$ is called the angle of incidence and the angle $\theta_2$ is the angle of refraction. Snell's Law, which can be proved using calculus, states that
$$
\frac{\sin \theta_1}{\sin \theta_2}=\frac{v_1}{v_2}
$$
The ratio $\frac{v_1}{v_2}$ is called the index of refraction. Some values are given in the table shown to the right.
(FIGURE CAN'T COPY)
Brewster's Law If the angle of incidence and the angle of refraction are complementary angles, the angle of incidence is referred to as the Brewster angle $\theta_B$. The Brewster angle is related to the index of refractions of the two media, $n_1$ and $n_2$, by the equation $n_1 \sin \theta_B=n_2 \cos \theta_B$, where $n_1$ is the index of refraction of the incident medium and $n_2$ is the index of refraction of the refractive medium. Determine the Brewster angle for a light beam traveling through water (at $20^{\circ} \mathrm{C}$ ) that makes an angle of incidence with a smooth, flat slab of crown glass.