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Azhar Zacharia
Asked: 2022-11-09T13:31:48+05:30 In:

An object placed at 20cm in front of a concave mirror produces three times magnified real image. What is the focal length of the concave mirror?
A. 15cm
B. 6.6cm
C. 10cm
D. 7.5cm

An object placed at 20cm in front of a concave mirror produces three times magnified real image. What is the focal length of the concave mirror?
A. 15cm
B. 6.6cm
C. 10cm
D. 7.5cm
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1 Answer

  1. Correct Answer – A
    We know that, linear magnification, `m=(f)/(f-u)`
    Given object displaced, u=-20cm m=-3 (`therefore “all images aer inverted”`)
    `So, -3=(f)/(f-(-20))`
    `-3=(f)/(f+20) Rightarrow -3f-60=f`
    `4f=-60 Rightarrow f=-(60)/(4)=-15cm`
    Since, mirror is concace, f=-15cm

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