Which material is cured by light to harden restorative resin?

• A. Gold alloy
• B. Amalgam
• C. Composite resin
• D. Zinc oxide eugenol

Answer: (C)

Light curing relies on a photoinitiator in the resin that absorbs blue light and generates free radicals to start polymerization. This causes the liquid resin to harden into a solid matrix, allowing the clinician to shape and finish the restoration in the same visit. Composite resin is specifically designed for this process, so it is cured by light to become a durable restoration. The other materials don’t use light to set: gold alloy is a metal used in casting and shaping, amalgam sets through a chemical reaction between mercury and alloy particles, and zinc oxide eugenol hardens by a chemical setting reaction.