1.

A convex lens of focal length 40cm is in contact with a concave lens of focal length 25cm. The power of the combination isA. `-1.5` dioptresB. `-6.5` dioptresC. `+6.5` dioptresD. `+6.67` dioptres

Answer» Correct Answer - A
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))=(1)/(0.4)+(1)/(-0.25)=(1)/(0.4)-(1)/(0.25)`
`(0.25-0.4)/(0.4xx0.25)=(-0.15)/(0.4xx0.25)=(-0.15)/(0.4xx0.25)=-1.5`
`rArrP=(1)/(f)=-1.5D`


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