The $x-z$ plane separates two media $A$ and $B$ with refractive indices $\mu_1$ and $\mu_2$ respectively. $A$ ray of light travels from $A$ to $B$. Its directions in the two media are given by the unit vectors $\vec{r}_A = a\hat{i} + b\hat{j}$ and $\vec{r}_B = \alpha\hat{i} + \beta\hat{j}$ respectively,where $\hat{i}$ and $\hat{j}$ are unit vectors in the $x$ and $y$ directions. Then:

• A
  $\mu_1a = \mu_2\alpha$
• B
  $\mu_1\alpha = \mu_2a$
• C
  $\mu_1b = \mu_2\beta$
• D
  $\mu_1\beta = \mu_2b$