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Question

A point object is placed at 60 cm from the pole of a concave mirror of focal length 10 cm on the principal axis. If the object is shifted 1 mm towards the mirror along the principal axis, then find the shift in image.

A
115 mm away from the pole
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B
115 mm toward the pole
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C
125 mm away from the pole
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D
125 mm toward the pole
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Solution

The correct option is C 125 mm away from the pole
Assuming the length shifted as a linear object, the length of the image will be the shift in the image of the point object.
Given:
u=60 cm
f=10 cm
Lo=1 mm [assumed as object]
From mirror formula,
1v+1u=1f
1v160=110
1v=112
v=12 cm
For longitudinal magnification,
mL=v2u2
mL=(12)2(60)2=125
LiLo=125
Li1=125
Li=125 mm
Thus, image gets shifted by 125 mm away from pole as object is shifted toward the mirror.
Why this question?In concave mirror, as object move towards the mirror till focus,the image shift away from the mirror.

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