Two thin convex lenses, each of focal length 20 cm, are placed coaxially 10 cm apart in air. The equivalent focal length of the combination is

1. For two thin lenses separated by distance d: 1/F = 1/f₁ + 1/f₂ − d/(f₁ f₂). 2. 1/F = 1/20 + 1/20 − 10/(20·20) = 0.05 + 0.05 − 0.025 = 0.075. 3. F = 1 / 0.075 ≈ 13.33 cm. _Source: syllabus Physics Part II, Ch 9 §9.5.4 (general two-lens formula)_