This little demo just shows an open-topped cylinder with different materials on its front and back faces. Two-sided lighting must be turned on to see the effect. When it is on, the cylinder is purple on the outside and yellow on the inside. With two-sided lighting off, the inside is also purple, but because the incorrect normal vector is used in the lighting equations, it is not correctly illuminated. In this case, only the rather dim ambient light is visible on the interior.
As usual, you can use your mouse to rotate the image.