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Question

An object is kept at a distance of 16 cm from a thin lens and the image formed is real. If the object is kept at a distance of 6 cm from the same lens, the image formed is virtual. If the sizes, of the images formed are equal, the focal length of the lens will be

A
15cm
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B
17cm
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C
21cm
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D
11cm
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Solution

The correct option is D 11cm
When object is kept 16 cm away,

1v11(16)=1f............ (1)

Magnification=v1(16)

When object is kept 6 cm away,

1v21(6)=1f................ (2)

Magnification=v26

Since the sizes of the images is same, M1=M2

Hence, v2=3v18

Using this relation in Equation 1 and Equation 2, we get
f=11cm

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