Practice question

**Lens maker's formula** 1/f = (n-1)(1/R₁ - 1/R₂), n refractive index, R₁,R₂ radii of curvature (m), sign convention R positive if surface convex towards incident light. For double convex R₁=12 cm, R₂=-12 cm, n=1.5, 1/f=(0.5)(1/12 -1/(-12))=(0.5)(2/12)=1/12, f=12 cm, converging. Lens maker’s formula: (1/f) = (n - 1) ( (1/R₁) - (1/R₂) ) . n = 1.6 , R₁ = 25 cm , R₂ = -25 cm . (1/f) = (1.6 - 1) ( (1/25) - (1/-25) ) = 0.6 ( (1/25) + (1/25) ) = 0.6 × (2/25) = (1.2/25) . f = (25/1.2) ≈ 20.83 cm . Substituting values gives 20.8 cm, which matches