(a) A +3.0 D convex lens is placed in contact with a −1.25 D concave lens. Find the combination's focal length and describe it. (b) Two +5 D lenses are 10 cm apart; find the effective power. (c) What spectacles correct myopia needing f = −50 cm? Step 1: P = 3.0 − 1.25 = +1.75 D. F = 1/1.75 ≈ 0.571 m = 57.1 cm converging. The concave lens weakened the convex one — the principle of achromatic doublets (with matched dispersion, colours focus together). Step 2: P = 5 + 5 − d·P₁P₂ = 10 − 0.10×25 = 10 − 2.5 = 7.5 D. Separation reduced the total from 10 D to 7.5 D — spacing matters in instruments. Step 3: P = 1/(−0.50) = −2.0 D diverging lens. Negative power always means concave/diverging: it spreads the rays so the myopic eye's over-strong system focuses them on the retina.