Question

# A convex lens forms an image at the distance 40 cm on other side of the lens when an object which is placed at 40 cm away from it . The object is then moved 10 cm towards the lens then, where will be the image shift?

A
20 cm towards the lens.
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B
10 cm away from lens.
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C
10 cm towards the lens.
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D
20 cm away from lens.
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Solution

## The correct option is D 20 cm away from lens.Initially, Object distance from the lens, u=−40 cm Image distance from the lens, v=40 cm Image distance is equal to the object distance. This happens when object is at 2f distance from the lens. 2f=40 f=20 cm When the object is moved 10 cm towards the lens, new object distance is: u′=−(40−10) cm u′=−30 cm Applying lens formula, the new image distance v′ is 120=1v′−1(−30) v′=60 cm Shift in image is: Δv=v′−v Δv=60−40=20 cm Thus, the image shift is 20 cm away from the convex lens.

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