For the refraction of light through a prism

For deviation angle $\mathrm{\delta }$ to be minimum, we need 
i = e
$\begin{array}{l}\Rightarrow {\mathrm{r}}_1={\mathrm{r}}_2\\ \Rightarrow {\mathrm{r}}_1+{\mathrm{r}}_2=\mathrm{A}\\ \Rightarrow {\mathrm{r}}_1={30}^0\end{array}$

If r₁ = 30^o then ray AB must be parallel to base of prism for minimum deviation so there can be only single angle of deviation but for other deviation angles there can be two angle of incidence ($\mathrm{\theta }$₁, $\mathrm{\theta }$₃), the figure shows the variation of deviation angle with angle of incidence.

In general for small angled prism, we have 
$\mathrm{\delta }=(\mathrm{\mu }-1)\mathrm{A}$
Where we can see that as increases, also increases.