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Question

# A very small linear object of size 1 mm is kept at a distance of 40 cm from a concave mirror of focal length 30 cm. The length of the image when the object is placed along the principal axis will be

A
9 mm
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B
6 mm
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C
4 mm
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D
2 mm
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Solution

## The correct option is A 9 mmGiven: Lo=1 mm u=−40 cm f=−30 cm So, 1v+1u=1f ⇒1v−140=−130 ⇒1v=−1120 ⇒v=−120 cm For longitudinal magnification, mL=−v2u2 ⇒mL=−(−120)2(−40)2=−9 ⇒LiLo=|−9| ⇒Li1=9 ⇒Li=9 mm Why this question?Since the length of object is small as compared to the the distance at which it is kept, therefore the formula for longitudinal magnification can be used.

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