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 :
Power of second lens is

Options

A. `-3.75 D`
B. `0.5 D`
C. `-5 D`
D. `1.25 D`

Answer

A

Explanation

`P_(2) = P - P_(1) = (100)/(F) - (100)/(f_(1))`
`:. P_(2) = (100)/(80) - (100)/(20) = 12.5 - 5 = - 3.75 D`