Two equilateral-triangular prisms \( P_1 \) and \( P_2 \) are kept with their sides parallel to each other, in vacuum, as shown in the figure. A light ray enters prism \( P_1 \) at an angle of incidence \( \theta \) such that the outgoing ray undergoes minimum deviation in prism \( P_2 \). If the respective refractive indices of \( P_1 \) and \( P_2 \) are \( \sqrt{\frac{3}{2}} \) and \( \sqrt{3} \), then \( \theta = \sin^{-1}\!\left[ \sqrt{\frac{3}{2}} \sin\!\left( \frac{\pi}{\beta} \right) \right] \), where the value of \( \beta \) is _______.
