$${\sin \varphi_2 - \sin \varphi_1 = 2 \cos \frac{\varphi_2 + \varphi_1}{2} \sin \frac{\varphi_2 - \varphi_1}{2} = \cos \varphi \Delta \varphi = \frac{\lambda_2 - \lambda_1}{d}}$$