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A concave mirror of radius of curvature 40 cm forms a real image twice as large ...


Question

A concave mirror of radius of curvature 40 cm forms a real image twice as large as the object.

The object distance is

Options

A) 10 cm

B) 30 cm

C) 40 cm

D) 60 cm


The correct answer is B.

Explanation:

if r = 40 cm , then f = r/2 = 40/2 = 20 cm
then for a real image by a concave mirror,
magnification M = f/u-f => 2 = 20/U-20
therefore 2(u-20) = 20
2u - 40 = 20
2u = 40 + 20
2u = 60/2 = 30 cm

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Dicussion (1)

  • if r = 40 cm , then f = r/2 = 40/2 = 20 cm
    then for a real image by a concave mirror,
    magnification M = f/u-f => 2 = 20/U-20
    therefore 2(u-20) = 20
    2u - 40 = 20
    2u = 40 + 20
    2u = 60/2 = 30 cm

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