A convex lens (tū tòu jìng) is a transparent optical device that is thicker at the center than at the edges, causing parallel light rays to converge at a focal point. The character 凸 means 'convex' or 'protruding', describing its outward-curving shape; 透 means 'to transmit' or 'to penetrate', indicating its function of allowing light to pass through; and 镜 means 'lens' or 'mirror', specifying it as an optical instrument. Together, the term literally conveys 'a protruding, light-transmitting lens'.

This word is commonly used in physics education, eyewear prescriptions (e.g., for farsightedness), magnifying glasses, microscopes, telescopes, and camera lenses. It contrasts with 凹透镜 (āo tòu jìng, concave lens), which diverges light. While technical, 凸透镜 appears in middle-school science textbooks and medical contexts — especially when discussing vision correction or optical instruments in clinical settings.