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NEET Sample Paper NEET Sample Test Paper-22

  • question_answer
    An object is placed at a distance of 10 cm from a co-axial combination of two lenses A and B in contact. The combination forms a real image three times the size of the object. If lens B is concave with a focal length of 30 cm, what is the nature and focal length of lens A?

    A)  Convex, 12 cm     

    B)  Concave, 12 cm        

    C)  Convex, 6 cm  

    D)      Convex, 18 cm

    Correct Answer: C

    Solution :

    \[m=-\frac{v}{u}=-3\]or \[v=3u\]  [\[\therefore \]The image is real the combination is convex] Now, u = 10 cm, therefore v = 30 cm. If F is the focal length of the combination, \[\frac{1}{F}=\frac{1}{v}+\frac{1}{u}=\frac{1}{30}+\frac{1}{10}\]or \[1=15/2\,cm.\] Again from ?\[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\] we get \[{{f}_{2}}=6\,cm.\] Hence, the correction option is [c].


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