text: Polymeric materials like plastic are made up of big molecules, and these very long molecules are disposed in a chaotic way, and there are links between molecules, It's something similar to a spaghetti dish and this structure is defined amorphous because there is no order. When plastic is put under stress these molecules are stretched and in that places the structure become more tidy , so it became crystalline, and crystalline structures are opaque, while amorphous structures are transparent.\nPigments are not important in this phenomenon.