Two converging lenses of focal length 20 ~cm and 40 ~cm are placed in contact. The effective power of the combination is

Step-by-step solution

The power of a lens is given by P = 1 f , where f is the focal length in meters. For two thin lenses in contact, the effective power P is the sum of the individual powers P₁ and P₂ . Given f₁ = 20 cm = 0.2 m and f₂ = 40 cm = 0.4 m . P₁ = 1 0.2 = 5 D P₂ = 1 0.4 = 2.5 D The effective power P = P₁ + P₂ = 5 D + 2.5 D = 7.5 D . Answer: 7.5 D