We know that the angle of deviation depends upon the angle of incidence. If we determine experimentally, the angles of deviation corresponding to different angles of incidence and then plot $\mathrm{i}$ (on $\mathrm{x}$-axis) and $\mathrm{\delta }$ (on $\mathrm{y}$-axis) we get a curve as shown in the figure.




Clearly, if the angle of incidence is gradually increased from a small value, the angle of deviation first decreases, becomes minimum for a particular angle of incidence and then begins to increase.