Two thin lenses of focal lengths 20 cm and 25 cm are placed in a contact. The effective power of the combination is
Two thin lenses of focal lengths 20 cm and 25 cm are placed in a contact. The effective power of the combination is
A. 9D
B. 2D
C. 3D
D. 7D
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Correct Answer - A
We have, the effective power given by,
`P_("eff")=P_(1)+P_(2)`
Also, we know `P=(1)/(f)`
Hence, `P_("eff")=(1)/(f_(1))+(1)/(f_(2))=(1)/(0.20)+(1)/(0.25)=5+4=9D`
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