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Power of a lens , P =(1)/(f) . True/Fal...

Power of a lens , `P =(1)/(f)` .
True/False

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TRUE
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The power of lens is P=1/(f') where f is focal length of the lens. The dimensions of power of lens is

Power of a lens is a measure of the ability of the lens to converge the rays of light falling on it. Quantitatively, power of a lens = reciprocal of focal length of lens, i.e., P=(1)/(f) . If a lens happens to diverge the ray of light falling on it, its power is said to be negative. Thus, power of a convex lens is positive and power of a concave lens is negative. If P_1,P_2 are powers of two lenses held in contact with each other, the power of the combination is P=P_1+P_2 . Note that P_1,P_2 are to be added with proper sign. Read the above passage and answer the following question: (i) What is the SI unit of power? (ii) Focal length of a concave lens is 20 cm . What is its power ? (iii) What lessons of life do you learn from the relation P=P_1+P_2 ?

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 : 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`

  • 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`

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    The principal focus of a convex lens is virtual . True/False

    The focal length of a cancave lens is negative. True/False

    A convex lens is a diverging lens. True/False

    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

    If two power of a thick lense is P_(1) and that of a thin lens is P_(2) then

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