Gold coats JWST’s primary-mirror segments. The mirror segments are made from lightweight, stiff beryllium, then covered with a microscopically thin gold layer that reflects infrared light efficiently.
Webb’s primary mirror contains 18 hexagonal segments, which together form a collecting surface about 6.5 to 6.6 meters across. Gold was selected because Webb is designed primarily for infrared astronomy: compared with aluminum, gold provides excellent reflectivity across much of the red and infrared range used by the observatory.
The coating is applied by vacuum vapor deposition. Inside a vacuum chamber, a small quantity of gold is vaporized and deposited onto the mirror surface. The layer is roughly 100 nanometers thick, and a thin protective coating is added because pure gold is soft and vulnerable to scratching.
The golden appearance does not mean the telescope is covered in a thick layer of precious metal. The entire primary mirror uses only about 50 grams of gold—roughly the mass of a golf ball. The gold is an optical coating, not the mirror’s structural material; beryllium provides the strength and dimensional stability needed in cryogenic space.