Paraxial optics

Snell’s law reads \(n_1 \sin{\theta_1} = n_2 \sin{\theta_2}\). In the paraxial regime this becomes \(n_1 \theta_1 = n_2 \theta_2\). Also, because in the paraxial regime \( \tan{\theta} = \theta \), \(\theta_1\) equals \(h/d\), so that \[\theta_2 = \frac{n_1 h}{n_2 d}\].