What is a triangular bar of glass with flat sides called?

A triangular bar of glass with flat sides is referred to as a prism. This is because a prism is defined as a solid geometric figure that has two parallel bases that are congruent polygons, and its other faces (the lateral faces) are parallelograms. In the case of a triangular prism, the two bases are triangles, and the sides are rectangular or parallelogram-like.

Prisms are significant in optics as they can bend or disperse light due to their geometry. When light passes through a prism, it may be separated into its constituent colors, creating a spectrum, which is a key concept in the study of light and optics.

The other options do not fit the description of a triangular bar of glass. A lens is typically curved and is not a solid with flat sides like a prism. A cube is a six-faced three-dimensional shape with all square faces, which does not align with the shape described. A pyramid is characterized by a base that can take various forms and triangular faces that lead to a single apex, which again differs from the properties of a prism.