noun
concave mirror
A mirror with an inwardly curved surface, used in telescopes, shaving mirrors, and other optical devices.
凹面鏡は光を一点に集める。
A concave mirror focuses light to a single point.
天体望遠鏡には凹面鏡が使われている。
Concave mirrors are used in astronomical telescopes.
平面鏡 is a flat mirror, whereas 凹面鏡 has a curved, inward surface.
Compound of 凹 (concave) + 面 (surface) + 鏡 (mirror). The reading is on'yomi.