Power of a combination of lenses in contact or separated

1. A.
   $20\text{ }cm$
2. B.
   $50\text{ }cm$
3. C.
   $500\text{ }cm$
4. D.
   $25\text{ }cm$

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1. A.
   5 D
2. B.
   1 D
3. C.
   20 D
4. D.
   10 D

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1. A.
   behaves as concave lens if $\left| f_{\text{convex~}} \right| > \left| f_{\text{concave~}} \right|$
2. B.
   behaves as concave lens if $\left| f_{\text{convex~}} \right| < \left| f_{\text{concave~}} \right|$
3. C.
   behaves as convex lens if $\left| f_{\text{convex~}} \right| > \left| f_{\text{concave~}} \right|$
4. D.
   Focal length of the lens system will change if the position of two lenses are interchanged

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