Google search didn't show up. It just shows up information related to spherical mirrors everywhere.
Is there a way to intuitively (and maybe formally) define convex (and concave) surfaces around a point $P$?
The following is an extract from my book:
"Now, the surface $S$ being convex, the quantity $\cos (\hat{r},\hat{n})$ is positive for every element $d\omega$ and can not become null in any of them."
Here the author is referring to the solid angle formula at point $P$ due to a surface $S$:
$$dS=\dfrac{r^2}{\cos (\hat{r},\hat{n})}\ d\omega$$
where the symbols have their their usual meanings.