Practice question

**Lens formula** 1/f = 1/v - 1/u, f focal length (m), u object distance, v image distance, sign: u negative if object left of lens in Cartesian convention, magnification m = v/u, real image inverted m negative, virtual erect m positive. Power P = 1/f (diopters D), f in meters, +5 D means f=0.2 m=20 cm converging. Focal length: f = 20 cm . Image distance: v = 60 cm (real image). Lens formula: (1/v) - (1/u) = (1/f) . (1/60) - (1/u) = (1/20) ⇒ (1/u) = (1/60) - (1/20) = (1 - 3/60) = (-2/60) = (-1/30) . u = -30 cm .