1.

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

Answer»

-1
-2
`+5`
`+2`

Solution :`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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