What is a Prism?

   In geometry, a prism is a solid object the sides of which are equal polygons and the ends equal and parallel polygons. Some prisms are given special names according to the shapes of their bases. Some of the most common shapes are triangular, quadrangular and rhombic. These names also apply to some crystal structures which grow in the shape of prisms.
   Probably the most common use for prisms is in the study of optics. One of the most interesting and simple applications of a triangular glass prism is to the study of the different wavelengths of light. If the prism is placed in a narrow beam of sunlight, the light will be bent as it passes through the prism. Not all of the colors will be bent in the same amount, however, so that a strip of colors will fall on a screen. This strip of colors is called the SPECTRUM and this method of producing it was discovered by Newton. The violet end of the spectrum is bent through a greater angle than the red end. The separation of the colors by this method is known as dispersion.
   A rather specialized application of prisms in optics is the polarization of light. The prism employed in this process is called a Nicol prism, named after its inventor, W. Nicol. The prism is made of two pieces of calcite cut in a particular manner and cemented together with Canada balsam. Because the index of refraction of Canada balsam is between the indices of refraction for the two pieces of the prism, some of the light (extraordinary ray) passes through the prism and the rest of the light (ordinary ray) is reflected out the side. Hence, the light coming out the end is polarized.