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Two lenses of focal lengths -20 cm and +...

Two lenses of focal lengths -20 cm and + 10 cm are conneceted to form a combination, their combinational power will be ...... D.

A

-1

B

-2

C

`+5`

D

`+2`

Text Solution

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The correct Answer is:
C
`f_1=-20` cm,`f_2=+10` cm
`therefore 1/F=(1)/(f_1)+(1)/(f_2)=(1)/(-20)+(1)/(10)`
`therefore 1/F=(1)/(20) therefore F=20` cm
Also,power P=`(100)/(F)=(100)/(20)=+5D`
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