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# A numerical Problem from optics

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??

Ans:

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

or

we get f = –20 cm

In II case

m=-1/3

v=u/3

f=-20cm

substituting in

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

## 1 Comment

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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