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Power or focal length of two thin lenses in contact

This type of question, asked 5 times: 2 May 2023 S2, 9 May 2023 S2, 10 May 2023 S2, 16 May 2023 S2, 10 May 2024 S2. All 5 questions below, oldest first.

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  1. Question 1

    2 May 2023 S2

    23
    The focal length of combination of lenses formed with lenses having power of +2.50 D and -3.75 D will be

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  2. Question 2

    9 May 2023 S2

    13
    Concave and convex lenses are placed touching each other. The ratio of magnitudes of their power is 2:3. The focal length of the system is 30 cm. The focal lengths of individual lens are

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  3. Question 3

    10 May 2023 S2

    14
    A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of combination is

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  4. Question 4

    16 May 2023 S2

    8
    An optician makes spectacles having a combination of a convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm. The power of this combination of lenses in dioptre is

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  5. Question 5

    10 May 2024 S2

    2
    Two lenses of power −15 D-15\,D and +5 D+5\,D are in contact with each other. The focal length of the combination is:

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