The figure shows how the angle of deviation $\delta$ changes with the angle of incidence $i$ for a glass prism of refracting angle $60^\circ$. The point P marks a ray with $i = 40^\circ$ and $\delta = 36^\circ$. In this graph a second ray also has a deviation of $36^\circ$, but it enters at a different angle of incidence. What is that angle of incidence?

Fig. 1: A graph of the angle of deviation, in degrees, on the vertical axis against the angle of incidence, in degrees, on the horizontal axis, for a glass prism of refracting angle 60 degrees. The curve falls from the left to a shallow lowest point and then rises again to the right. A point P on the falling part of the curve is marked with dashed lines to the axes, at an angle of incidence of 40 degrees and a deviation of 36 degrees.