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Question

A particle goes is in the form of a circle of radius 2.0 cm. A concave mirror of focal length 20 cm is placed with its principal axis passing through the centre of the circle and perpendicular to its plane. The distance between the pole of the mirror and the centre of the circle is 30 cm. Calculate the radius of the circle formed as the image.

A
2 cm
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B
8 cm
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C
4 cm
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D
5 cm
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Solution

The correct option is C 4 cm
Given u=30 cm,f=20 cm
From mirror formula,
1v+1u=1f
1v+(130)=(120)
v=60 cm.

Image of the circle is formed at a distance 60 cm in front of the mirror.
m=vu=RimageRobject
m=6030=Rimage2

Rimage=4 cm

Radius of image of the circle is 4 cm


Why this Question?

Tip: when object is placed at a distance from principle axis, image position is calculated using magnification formula.

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