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A thin convex lens (L(1)) of focal lengt...

A thin convex lens `(L_(1))` of focal length 80 cm and a thin concave lens `(L_(2))` of focal length f are kept co-axially, 20 cm apart as shown in figure below. When a narrow and parallel beam of light is incident on the conver lens, beam emerging from the concave lens is also a parallel beam. Find f.

Text Solution

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Parallel rays after refraction through convex lens will converge at focus. So, convex lens will focus at 80 cm.
So, object for concave lens will be at 60 cm from it on the right hand side.
We know, `(1)/(f)=(1)/(v)-(1)/(u)`
`(1)/(f)=(1)/(oo)-(1)/(60)`
`impliesf=-60cm`.
So, focal length of concave lens is 60 cm.
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