A glass prism is able to produce the spectrum when white light passes through it, but a glass slab doesn’t produce any spectrum. Explain why it is so.
The two faces of a glass prism are at an angle (angle of prism) with each other. Therefore, the angle of incidence and the angle of emergence of a light ray become different in the case of a prism. Therefore, when white light gets dispersed after entering a prism, its component colours spread more after refracting through the opposite face of the prism.
In the case of a glass slab, the two opposite faces of the slab are parallel to each other. Therefore, the angle of incidence and the angle of emergence of a light ray remain the same for a glass slab. Therefore, the dispersed light rays combine to form white light after refracting through the second face of the glass slab.