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By what factor will the size of an object placed 10cm from a convex lens be incr...


Question

By what factor will the size of an object placed 10cm from a convex lens be increased if the image is seen on a screen placed 25cm from the lens?

Options

A) 15.0

B) 2.5

C) 1.5

D) 0.4


The correct answer is B.

Explanation:

Form the lens formula = 1/U + 1/V = 1/f
=>1/10 + 1/25 = 1/f;
∴ (5 + 2) / 50 = 1/f
∴ f = 50/7cm
since the image is observed on a screen, it is a real image. Thus magnification
M = f/U-f
=__50/7
10 - 50/7
∴ M =50/7
20/7

= 50/7 x 7/20 = 2.5

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Dicussion (1)

  • Form the lens formula = 1/U + 1/V = 1/f
    =>1/10 + 1/25 = 1/f;
    ∴ (5 + 2) / 50 = 1/f
    ∴ f = 50/7cm
    since the image is observed on a screen, it is a real image. Thus magnification
    M = f/U-f

    =__50/7
    10 - 50/7
    ∴ M =50/7
    20/7

    = 50/7 x 7/20 = 2.5

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