1.

The plano-convex lens of focal length 20cm and 30cm are placed together to form a double convex lens.The final length will be

Answer»

`12cm`
`60CM`
`20cm`
`30cm`

Solution :(a) EQUIVALENT focal length
`1/F=1/(f_(1))+1/(f_(2))=1/20+1/30`
`F=(20xx30)/(20+30)=600/50=12cm`


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