Power `(P)` of a lens is given by reciprocal of focal length `(f)` of the lens. i.e. `P = 1//f`. When `f` is in metre, `P` is in dioptre. For a convex lens, power is positive and for a concave lens, power is negative. When a number of thin lenses of powers `p_(1), p_(2), p_(3)....` are held in contact with one another, the power of the combination is given by algebraic sum of the powers of all the lenses i.e., `P = p_(1) + p_(2) + p_(3) + ........` Answer the following questions : Two thin lenses are in contact and the focal length of the combination is `80 cm`. If the focal length of one lens is `20 cm`, the focal length of the other would beA. `-26.7 cm`B. `60 cm`C. `80 cm`D. `20 cm`

Power `(P)` of a lens is given by reciprocal of focal length `(f)` of

1.	Power `(P)` of a lens is given by reciprocal of focal length `(f)` of the lens. i.e. `P = 1//f`. When `f` is in metre, `P` is in dioptre. For a convex lens, power is positive and for a concave lens, power is negative. When a number of thin lenses of powers `p_(1), p_(2), p_(3)....` are held in contact with one another, the power of the combination is given by algebraic sum of the powers of all the lenses i.e., `P = p_(1) + p_(2) + p_(3) + ........` Answer the following questions : Two thin lenses are in contact and the focal length of the combination is `80 cm`. If the focal length of one lens is `20 cm`, the focal length of the other would beA. `-26.7 cm`B. `60 cm`C. `80 cm`D. `20 cm`
Answer» Correct Answer - A Here, `f_(1) = 20 cm. f_(2) = ? F = 80 cm`. As `(1)/(f_(1)) + (1)/(f_(2)) = (1)/(F)` `:. (1)/(f_(2)) = (1)/(F) - (1)/(f_(2)) = (1)/(80) - (1)/(20) = -(3)/(80)` `f_(2) = -(80)/(3) = - 26.7 cm`

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