Practice question

**Two lenses in contact** behave as single lens with power sum, magnification product m = m₁×m₂. For f₁=20 cm (P₁=5 D), f₂=-20 cm (P₂=-5 D) in contact, P=0, F infinite, afocal system, beam emerges parallel, principle of corrective lenses for myopia/hypermetropia. Object distance: u = -5 cm (virtual object). f = 10 cm . (1/v) - (1/-5) = (1/10) ⇒ (1/v) + (1/5) = (1/10) ⇒ (1/v) = (1/10) - (1/5) = (1 - 2/10) = (-1/10) . v = -10 cm (10 cm to the left of the lens). Substituting values gives 10 cm, which matches expected image position and magnification from mirror/lens formula