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. f₁ = 30 cm , f₂ = -60 cm . (1/f) = (1/f₁) + (1/f₂) = (1/30) + (1/-60) = (2 - 1/60) = (1/60) . f = 60 cm (converging system). Substituting values gives 60 cm, which matches expected image position and magnification from mirror/lens formula 1/f = 1/v - 1/u (lens) or 1/f = 1/v + 1/u (mirror), confirming sign conventions and refraction principles.