Question

Why the phenomenon of dispersion of light ray is possible through a prism but not through a rectangular glass lab??

Answer 1

Unlike a prism an incident ray on a glass slab suffers no deviation from the emergent ray because the opposite sides of the glass slab are parallel. Therefore, none of the colors of the light is deviated but only refracted. Lack of deviation doesn't separate the individual colors of the light. Hence, there is no dispersion.