Practice question

**Spherical refracting surface** power P = (n₂-n₁)/R, lens power sum of two surfaces. Lens maker derivation combines two refractions, sign of R₂ negative for second surface convex opposite direction, yielding 1/f positive for convex lens. Object distance: u = -5 cm (virtual object), f = -20 cm . Lens formula: (1/v) - (1/-5) = (1/-20) ⇒ (1/v) + (1/5) = (1/-20) . (1/v) = (1/-20) - (1/5) = (-1 - 4/20) = (-5/20) = (-1/4) . v = -4 cm (4 cm to the left). Substituting values gives 4 cm, which matches expected image position and magnification from mirror/lens formula 1/f = 1/v - 1/u