Equivalent Focal Length Separated Lenses — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Equivalent Power of Separated Lenses This formula applies when two optical lenses are placed coaxially and separated by a distance d, assuming the lenses are thin and the medium is air. Define the power of a single lens as P = 1/f , where f is the focal length. Determine the image distance ( v 1 ) formed by the first lens ( L 1 ) when an object is placed at infinity (or at the first lens). Use the image distance v 1 as the object distance for the second lens ( L 2 ). Apply the lens formula sequentially and simplify the resulting expression to obtain the combined power P eq . d 0 P 1 and P 2 are the powers of the individual lenses (in Diopters). Treating the formula as simply additive ( P eq = P 1 + P 2 ), which is only true if the lenses are touching ( d=0 ). Confusing power ( P ) with focal length ( f ), forgetting that P = 1/f . Incorrectly applying the separation distance d (it must be multiplied by the product of the powers).