Question

two lens of power 2D and minus 2.5 d are placed in contact with each other the focal length of convex lens is - (1) -15 (2) -100 (3)- 150 (4) - 200

Answer 1

Answer:

Answer is 3)=-150.

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Answer 2

Answer:

its (d)

Explanation:

BECAUSE POWER OF COMBINATION OF LENS =P1+P2+P3+..........

Here,P1=2Dand P2=-2.5D

so power of combination of lens=P1+P2

                                                     =2+(-2.5)

                                                     =-0.5D

so focal length =1/P

                         =1/-0.5

                         =-2m

                         =-200cm

hope it helps..