Question

equivalent focal length of two lenses in contact having power -15D and +5D will be

Answer 1

Net power=P1 + P2
= -15D + 5D
= -10D
Ans. -10D
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Answer 2

F1 = -1/15 m

F2 = 1/5 m

1/F = 1/F1 + 1/F2

Or, 1/F = -1/15 + 1/5

Or, 1/F = (-1+3)/15

Or, F = 15/2

Or, F = 7.5 m

So the equivalent focal length is 7.5 m

Focal length is the distance between the centre of a lens or curved mirror and its focus.