A Prism

1. (geometry) A polyhedron with parallel ends of the same size and shape, the other faces being parallelogram-shaped sides. Prism (geometry) - Wikipedia
2. A transparent block in the shape of a prism (typically with triangular ends), used to split or reflect light. Prism - Wikipedia
3. A crystal in which the faces are parallel to the vertical axis.

Prism Dispersing:

When a white light passes through a prism, light of different colors (wavelengths) will emerge at different angles with respect to the initial beam of light.

If white light falls all over a prism, the red from one part will overlap the blue from another part, so the spectrum will be unclear.