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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 :
Focal length of the combined three lenses would be

A

`-26.7 cm`

B

60 cm

C

80 cm

D

20 cm

Text Solution

Verified by Experts

The correct Answer is:
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_1)=(1)/(80)-(1)/(20)= -(3)/(80)`
`f_2 = -(80)/(3) = -26.7 cm`.
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