Light boxes work by taking advantage of the fact that most paper is not entirely opaque. Standard printer paper, for example, is slightly translucent when exposed to bright light. When a blank page of paper is placed in front of another piece of paper, and both sheets of paper are placed in front of a light source, the design on the second piece of paper (in the back) is visible from the front of the first piece of paper. When the light source is flat and horizontal – such as this homemade lightbox or various professional options, this can be a tool for professional and amateur artists to copy or trace designs.