Question

how does the dispersive power of glass prism change when it is dipped in water?

Answer 1

Angle of minimum deviation = Dm = (μ - 1) A, where A is the angle of prism.

μ = refractive index of glass of prism wrt to air  or water as the case may be.

Refractive index of glass wrt water is less than wrt to air.  Hence angle of deviation is less.  Hence dispersive power is less.