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Power (P) of a lens is given by reciproc...

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

A

`- 3.75 D`

B

`3.75 D`

C

`5.0 D`

D

`- 5.0 D`

Text Solution

Verified by Experts

The correct Answer is:
A
Here, `P = P_1 + P_2 = (100)/(F) = (100)/(80) = 1.25 D`
`P_3 = (100)/(f_3) = (100)/(-20) = -5 D`
`:.` Power of the combination of three lenses is
`P' = P_1 + P_2 + P_3 = 1.25 - 5`
=`-3.75 D`.
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