Answer
$n_1sin\theta_1=n_3sin\theta_3$
Work Step by Step
According to Snell's law
$n_1sin\theta_1=n_2sin\theta_2$..........eq(1) $(n_2\gt n_1)$
Now the angle of refraction $\theta_2$ acts as an incident ray for the second medium $n_2$ and the angle of refraction becomes $\theta_3$ in medium ($n_3$)
Thus $n_2sin\theta_2=n_3sin\theta_3$......eq(2) $n_3\gt n_2$
From eq(1) and eq(2), we have
$n_1sin\theta_1=n_3sin\theta_3$
