Practice question

**Lens combination** effective focal length calculation uses reciprocal sum, for f₁=10 cm, f₂=20 cm in contact, 1/F=1/10+1/20=3/20, F=6.67 cm, P=15 D, stronger converging than either alone, illustrating power addition. Focal length: f = 20 cm (convex mirror). Object distance: u = -40 cm . Mirror equation: (1/v) + (1/u) = (1/f) . (1/v) + (1/-40) = (1/20) ⇒ (1/v) = (1/20) + (1/40) = (2 + 1/40) = (3/40) . v = (40/3) ≈ 13.33 cm (virtual image). Substituting values gives 10 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