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

A concave mirror of radius of curvature 40 cm forms a real image twice as large as the object.
the object distance is
  • A.
    10 cm
  • B.
    30 cm
  • C.
    40 cm
  • D.
    60 cm
Correct Answer: Option 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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