Applying Snell's law at B and C,

$\mu \sin i=$constant or ${\mu }_1{\mathrm{sini}}_{\mathrm{B}}={\mu }_4\text{ sub }{\mathrm{i}}_{\mathrm{c}}$

But since $A B\left|\right|C D$

$\therefore i_B=i_C$

or ${\mu }_1={\mu }_4$

The Snell's law, which the students read in their plus two syllabus $\mu =\frac{\sin i}{\sin r}$comes from $\mu \sin i=$constant

${\mu }_1={\mu }_1{\mathrm{sini}}_1={\mu }_2{\mathrm{sini}}_2$

${\mu }_2=\mu$ or $\mathrm{sini}=\mu \mathrm{sinr}$

or $\mu =\frac{\sin i}{\sin r}$