The focal length of a concave mirror is 20 cm. An object is placed at 30 cm from the mirror. The image distance is: MCQ with Answer and Explanation

Mirror formula: 1/f = 1/u + 1/v. Sign convention: f negative for concave mirror? Wait, standard Cartesian: f = -20 cm (concave), u = -30 cm (object real). Thus 1/(-20) = 1/(-30) + 1/v ⇒ -1/20 = -1/30 + 1/v ⇒ 1/v = -1/20 + 1/30 = (-3+2)/60 = -1/60 ⇒ v = -60 cm. Magnitude 60 cm, negative sign indicates real image in front of mirror. Many exams use magnitude-only options; here 60 cm is correct. Memory tip: 'Concave mirror: object beyond C (2f) ⇒ image between F and C, real, inverted'. This numerical tests mirror formula application with sign conventions, crucial for optics in competitive exams.