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

Options

A. `-1.5` dioptres
B. `-6.5` dioptres
C. `+6.5` dioptres
D. `+6.67` dioptres

Answer

A

Explanation

`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))=(1)/(0.4)+(1)/(-0.25)=(1)/(0.4)-(1)/(0.25)`
`(0.25-0.4)/(0.4xx0.25)=(-0.15)/(0.4xx0.25)=(-0.15)/(0.4xx0.25)=-1.5`
`rArrP=(1)/(f)=-1.5D`