Question

# A convex lens forms an image of an object on a screen. The height of the image is 9 cm. The lens is now displaced until an image is again obtained on the screen. The height of this image is 4 cm. The distance between the object and the screen is 90 cm.

A
The distance between the two positions of the lens is 30 cm.
B
The distance of the object from the lens in its first position is 36 cm.
C
The height of the object is 6 cm.
D
The focal length of the lens is 21.6 cm.
Solution
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#### h2object=himage1×himage2hobject=√36=6magnification of image is vu=96 v=3u2in lens displacement method , u+v=d ; uv=dfu+3u2=90u=36 => v=54uv=df f=36×5490=21.6option D is correct

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