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 :
When a third lens of focal length `- 20 cm` is placed in contact with the two lenses, power of the three would be

Options

A. `-3.75 D`
B. `3.75 D`
C. `5.0 D`
D. ( d) `-5.0 D`

Answer

A

Explanation

Here, `P = P_(1) + P_(2) = (100)/(F) = (100)/(80) = 1.25 D`
`P_(3) = (100)/(f_(3)) = (100)/(-20) = - 5D`
`:.` power of the combination of three lenses is
`P' = P_(1) + P_(2) + P_(3) = 1.25 - 5 = - 3.75 D`