A glass prism ' A ' deviates the red and blue rays through $10^{\circ}$ and $12^{\circ}$ respectively. A second prism ' B ' deviates them through $8^{\circ}$ and $10^{\circ}$ respectively. The ratio of their dispersive powers is (A to B)

The angle of minimum deviation produced by a thin prism in air is $\delta_1$. If it is immersed in water the angle of minimum deviation is

$$\left[\mathrm{a}_{\mathrm{g}}=\frac{3}{2}, \mathrm{a}_{\mathrm{w}}=\frac{4}{3}\right]$$

For a symmetric (equilateral) prism, the prism formula can be written as

For a ray of light, the critical angle is minimum, when it travels from