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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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Knowledge Check

  • 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
    `0.5 D`
    C
    `-5 D`
    D
    `1.25 D`

  • 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 be

    A
    `-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 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

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

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