Two Thin Lenses Are Placed 5 Cm Apart Along The Same Axis And Illuminated With A Beam Of Light Parallel To That Axis. The First Lens In The Path Of The Beam Is A Converging Lens Of Focal Length 10 Cm Whereas The Second Is A Diverging Lens Of Focal Length 5 Cm. If The Second Lens Is Now Moved Toward The First, The Emergent Light  

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Question

Two thin lenses are placed 5 cm apart along the same axis and illuminated with a beam of light parallel to that axis. The first lens in the path of the beam is a converging lens of focal length 10 cm whereas the second is a diverging lens of focal length 5 cm. If the second lens is now moved toward the first, the emergent light  

Solution

Correct option is

Changes from parallel to divergent

                                                    

 

At 5 cm from the lens, the second lens has a virtual object (image of the first lens) at its focal length. The emergent rays are therefore parallel.

                                                                 
When the second lens is closer than 5 cm to the fist lens, its object is outside the focal length of the diverging second lens. This produces a virtual image outside 2f of the second lens. The emergent rays are therefore divergent.   

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