Question

A convex lens of power +6 D is in contact with a concave lens of power -4 D. What will be the focus length and nature of the combined lens?
(a) Concave, 25 cm
(b) Convex, 50 cm
(c) Concave, 20 cm
(d) Convex, 100 cm

Answer 1

(b) Convex, 50 cm
P₁ = +6D, P₂ = -4 D, P = ?, f = ?
P_eff = P₁ + P₂ = 6 – 4 = +2D
∴ Focal length of combined lens
f = \(\frac{100}{P}=\frac{100}{+2}\) = +50 cm