An equilateral prism deviates a ray through $40^{\circ}$ for two angles of incidence differing by $20^{\circ}$. The possible angles of incidence are:

For a thin prism,$\delta_1$ is the angle of deviation produced when the prism is placed in air. When the prism is immersed in water,the angle of deviation produced is $\delta_2$. Given ${ }_{a} \mu_{g}=\frac{3}{2}$ and ${ }_{a} \mu_{w}=\frac{4}{3}$. The ratio $\delta_2: \delta_1$ is

$A$ light ray is incident normally on one surface of an equilateral prism. The angle of deviation of the light ray is (refractive index of the material of the prism $=\sqrt{2}$) (in $^{\circ}$)

In the position of minimum deviation when a ray of yellow light passes through a prism,its angle of incidence is

$A$ ray of light is incident normally on the first refracting face of a prism of refracting angle $A$. The ray of light comes out at grazing emergence. If one half of the prism (shaded portion) is knocked off,the same ray will:

Angle of minimum deviation is equal to the half of the angle of prism in an equilateral prism. The refractive index of the prism is . . . . . . .