$$\require{cancel}$$
In Figure II.13 shows a reflecting surface of radius of curvature $$r$$ submerged in a medium of index $$n$$. I show a real object at O, a virtual image at I and the centre of curvature at C. We see that $$h = \alpha p = \beta q = \gamma r$$. By Euclid, $$\theta = \alpha + \gamma$$ and $$2\theta = \alpha + \beta$$. Remember again that all angles are supposed to be small (even $$\beta$$!), in spite of the drawing. From these we obtain
$\frac{1}{q} = \frac{1}{p} + \frac{2}{r}. \label{eq:2.8.4}$