I have the following figure from Fundamentals of Photonics, Third edition, by Saleh and Teich:
The accompanying explanation says the following:
The angle $\theta_2$ is negative since the ray is traveling downward. Since the three angles of a triangle add to $180^\circ$, we have $\theta_1 = \theta_0 - \theta$ and $(-\theta_2) = \theta_0 + \theta$, so that $(-\theta_2) + \theta_1 = 2 \theta_0$. ...
What geometric/trigonometric reasoning (besides the obvious fact that the angles of a triangle add to $180^\circ$) was used to deduce that $\theta_1 = \theta_0 - \theta$ and $(-\theta_2) = \theta_0 + \theta$? It seems to me to be some theorem of similar triangles with a bisector angle (although, it's not clear that the angles on either side of the line splitting $\theta$ are equal), but I'm unsure.
I would greatly appreciate it if someone could please take the time to clarify this.