An object is placed 30 cm from a concave mirror of focal length 20 cm. The image distance is

1. With your syllabus signs: u = −30 cm, f = −20 cm. 2. 1/v + 1/u = 1/f ⇒ 1/v = 1/f − 1/u = −1/20 + 1/30 = (−3 + 2)/60 = −1/60. 3. v = −60 cm — image is real (negative v in this convention) and 60 cm in front of the mirror. _Source: syllabus Physics Part II, Ch 9 §9.2.3 worked example pattern_