Question

What is the difference between the displacement produced by a rectangular glass Slab in the ray of light and the deviation produced by the glass prism

Answer 1

In the case of a glass slab, the two interfaces the light ray has to pass are parallel to each other. In this case the emergent ray is also a white light because the constituents of whitelight which are refracted at different angles at the first interface recombine at the second interface and emerge as a sigle white light.

In the case of a prism,  the two interfaces the light ray has to pass are not parallel to each each other and are inclined at an angle. In this case, the effects of the first interface are not reversed and the colors separated at that first interface continue along different paths upon leaving the glass at the second interface. Hence we observe a spectrum on the other sisde.

Answer 2

In glass slab two refracting surfaces of the glass slab are parallel to each other, the emergent ray is parallel to incident ray and hence the displacement occurs but it's direction is not changed.

Where as

In the prism, the incident ray and emergent ray are not parallel to each other. The emergent ray is deviated from the direction of the incident ray by the angle of deviation. And, therefore we can see the splitting up of light into its constituent colours on the other side.

hope it helps you!!