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A numerical Problem from optics

AK asks:

When an object is kept at a distance of 60cm from a concave mirror, the magnification is 1/2. Where should the object be placed to get magnification of 1/3??

please answer this question….


In the first part, u=-60, m=(-)1/2 implies v=-30 cm (Since the image must be real as concave lens form virtual image of bigger size only)

Substituting in eqn mirror_formula

or  mirrorformula

we get f = –20 cm

In II case




substituting in mirrorformula

we get u=4f=4 x (-20)=-80 cm

(If this is not the expected answer please respond)


  1. electricity question,
    Q.1- A heating element of a hot plate on an electric cooking range draws 12 amperes from 240 V mains. How many kwh of energy will be consumed in one hour and 15 minutes
    (A) 1.2
    (B) 3.2
    (C) 6.0
    (D) 7.2

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