This is a triangular prism. The triangular part refers to the
two triangles which are in parallel planes. The triangles are the bases of the prism.
Translucent prisms refract ordinary white light into the color
spectrum. In the diagram below, white light is directed toward the prism on the
left. As the light passes through the prism it is separated into the seven
colors of visible light: Red, Orange, Yellow, Green, Blue, Indigo, Violet (like a rainbow!). This
can be remembered with the acronym: ROY-G-BIV.