1.

Two thin lenses of focal lengths `f_(1)` and `f_(2)` are in contact. The focal length of this combination isA. `f_(1) + f_(2)`B. `(f_(1)f_(2))/(f_(1) + f_(2))`C. `(1)/(2) (f_(1) + f_(2))`D. `(f_(1) + f_(2))/(f_(1)f_(2))`

Answer» Correct Answer - D
Focal length of combination,
`(1)/(F)=(1)/(f_(1))+(1)/(f_(2))`
`F=(f_(1)f_(2))/(f_(1)+f_(2))`
Power of lens, `P=(1)/(F)=(f_(1)+f_(2))/(f_(1)f_(2))`


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