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Two lenses of power +15D are in contact ...

Two lenses of power +15D are in contact with each other. What is focal length of Combination?

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`P_(1)+15D`
`P_(2)=-5D`
Power of combination of lenss `=P=P_(1)`
`+P_(2)=+15-5=+10D`
So focal length of combination
`F=1/P=1/10=0.1m`
F=10cm
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