\frac{\sin { \theta }_{P1} }{ {P}_{1} } = \frac{\sin { \theta }_{P1} }{ {PP}_{r} } = \frac{\sin { \theta }_{P2} }{ {PP}_{t} } = \frac{\sin { \theta }_{S1} }{ {PS}_{r} } = \frac{\sin { \theta }_{S2} }{ {PS}_{t} }
