A convex lens of focal length 20 ~cm and a concave lens of focal length 40 ~cm are placed in contact with each other along their principal axis. The equivalent power of this combination is :

Step-by-step solution

The power of a lens is given by P = 100 f( in cm ) . For the convex lens, the focal length is positive: P₁ = 100 +20 = +5 D For the concave lens, the focal length is negative: P₂ = 100 -40 = -2.5 D The equivalent power of the combination is the algebraic sum of the individual powers: P_ eq = P₁ + P₂ P_ eq = +5 D + (-2.5 D ) = +2.5 D Answer: +2.5 D