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Question

Parallel rays of light are focussed by a thin convex lens. A thin concave lens of same focal length is then joined to the convex lens and the result is that :

A
the focal point shifts away from the lens by as mall distance
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B
the focal point shifts towards the lens by as mall distance
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C
the focal point of lens does not shift at all
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D
the focal point shifts to infinity
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Solution

The correct option is D the focal point shifts to infinity

Parallel rays on convex lens (u=)

So, 1v1u=1f

1v1=1f+1u

1v1=1f

v1=f

Now for concave lens v1 becomes u

So, 1v1u=1f

1v=1f+1v1

(because concave lens has negative focal length)

1v=1f+1f (because focal length of both lenses are same )

1v=0

Or v=

So, image is formed at infinity or focal point of combination shifts to infinity.


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