Prism

A prism is a triangular block of glass or plastic, bounded by three plane faces that are not parallel to one another. One face, the ground (unpolished) face, is called the base of the prism; the other two, polished faces are the refracting faces. The angle between the two refracting faces is called the angle of the prism, the refracting angle, or the apex angle, usually denoted $A$. Light entering one refracting face and leaving through the other undergoes two refractions, one at each face, and the resulting change of direction -- studied throughout this section -- is what makes a prism useful both for producing a controlled, measurable devi …