A ray of light is incident in medium $1$ on a surface that separate medium $1$ from medium $2$. Let $v_1$ and $v_2$ represent the velocity of light in medium $1$ and medium $2$ respectively. Also let $n_{12}$ and $n_{21}$ represent the refractive index of medium $1$ with respect to medium $2$ and refractive index of medium $2$ with respect to medium $1$, respectively. If $i$ and $r$ denote the angle of incidence and angle of refraction, then: