Question

If a glass prism is dipped in water, its dispersive power
(a) increases
(b) decreases
(c) does not change
(d) may increase or decrease depending on whether the angle of the prism is less than or greater than 60°

Answer 1

(b) decreases

If μ is the refractive index and A is the angle of prism, then the angular dispersion produced by the prism will be given by $\delta =(\mu -1)A$.
Because the relative refractive index of glass with respect to water is small compared to the refractive of glass with respect to air, the dispersive power of the glass prism is more in air than that in water.