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Q.

A convex lens of focal length 30 cm and a concave lens of 10 cm focal length are placed so as to have the same axis.  If a parallel beam of light falling on convex lens leaves concave lens as a parallel beam, then the distance between two lenses will be            

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a

40 cm 

b

10 cm

c

20 cm

d

30 cm 

answer is C.

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Detailed Solution

Let distance between lenses be x.  

As per the given condition, combination behaves as a plane glass plate, having focal length = ∞

So by using 1F=1f1+1f2−xf1f2

⇒1∞=1+30+1-10-x(+30)(-10)⇒x=20 cm

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