A convex lens of focal length 25 cm is kept is contact with a concave lens of focal length 40 cm. What is the power of their combination ?

Options

A. 2.5 D
B. `-2.5 D`
C. `+1.5 D`
D. `-1.5 D`

Answer

C

Explanation

The equivalent focal length of the combination is
`(1)/(F)=(1)/(f_(1))+(1)/(f_(2))`
where `f_(1)=25cm` (convex) and `f_(2)=-40cm` (concave)
`(1)/(F)=(1)/(25)-(1)/(40)=(8-5)/(200)=(3)/(200)`
`therefore F=(200)/(3) cm =(2)/(3)m therefore " Power "=(1)/(F)=(3)/(2)D`