Acrylic (PMMA)

PMMA, short for polymethyl methacrylate, is the clear plastic usually called acrylic. It's about half as heavy as glass, much harder to shatter, and lets through about 92% of visible light. Aquarium walls, shop signs and car tail lights are often made of it.

It's built from a single small molecule, methyl methacrylate. Each molecule has a carbon–carbon double bond. A starter chemical called an initiator breaks open one double bond, and that molecule then grabs the next, which grabs the next, in a chain reaction. Thousands of molecules link into each long chain. For sheets, the liquid is poured between glass plates and slowly hardened. For molded parts, it's made into pellets that are melted and injected into steel molds.

On an LED strip in a direct-lit TV, each LED usually wears a small molded lens, often acrylic. On its own, the LED shines mostly straight up and makes a bright spot. The lens bends its light outward into a wide, even spread, so fewer LEDs can light the screen smoothly. Acrylic sheet went on sale in the 1930s and was used for aircraft canopies in World War II.