\magnification=\magstep1


$$
\sin(\alpha_1) \quad = \quad {{ v_1 \Delta t}\over{AB} }
$$


$$
\sin(\alpha_2) \quad = \quad {{ v_2 \Delta t}\over{AB} }
$$

$$
{{\sin(\alpha_1)}\over{\sin(\alpha_2)}} \quad = \quad {{v_1}\over{v_2}}
$$


$$
{\rm speed\ of\ wave\ } v_1 \quad = \quad {{v}\over{n_1}}
$$

$$
{{\sin(\alpha_1)}\over{\sin(\alpha_2)}} \quad = \quad {{n_2}\over{n_1}}
$$



\bye
