This is a good demonstration to show one of the applications of polarized light and to observe beautiful, colorful patterns. This simple method is employed by industrial manufacturers to assure quality control of plastic and glass products. The device (see picture to the left) consists of a light source and a matte screen for uniform illumination. A big polarizer is placed on top of the screen and can be rotated by hand, while a small polarizer can be slid up into place to accommodate a range of objects. Areas of stress will be revealed when an object is placed between the polarizers (see pictures below).