Create a device from a sheet of card and an old CD or DVD which splits light into its constituent rainbow. You can use it to determine the make up of the source of the light.

Cut around the dashed lines of the cross shaped box outline. Use a scalpel to cut the rectangle for the viewing hole, the slit to insert the CD, and another slit across the square to view the light source. Fold the tabs and box sides away from you and secure them to create a box. Insert the CD the correct way up (as shown on the box) and push gently until it touches the interior corner of the box.