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A convex lens of focal length 40 cm is i...

A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is ..........

A

`-6.5` D

B

`+6.5` D

C

`+1.5` D

D

`-1.5` D

Text Solution

Verified by Experts

The correct Answer is:
A, D
The power for the concave lens of focal length `f_1` = 25 cm
`P_1=(-1)/(f_1)=-(1)/(0.25)=-4D`
The power for the concave lens of focal length `f_2` = 40 cm
`P_2=(1)/(f_2)=(1)/(0.40)=2.5D`
Note : The focal length of concave lens is negative.
`therefore` The power of the combination of two lens,
`P=P_1+P_2=-4.0+2.5=-1.5D`
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