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Solution


Q.

An effective power of a combination of 5 identical convex lenses which are kept in contact along the principal axis is 25 D. Focal length of each of the convex lens is              [2024]
1 20 cm  
2 50 cm  
3 25 cm  
4 500 cm  

Ans.

(1)       

             We know that Peq=ΣPi

              As given all lenses are identical, 5P = 25 D

              ∴ P=5 D

             ⇒1f=5⇒f=15 m=20 cm