An eye specialist prescribes spectacles having combination of convex lens of focal length 40cm in contact with a concave lens of focal length 25cm. The power of this lens combination in diopters is

Options

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

Answer

B

Explanation

(b) `therefore` power of the lens=`1/"Focal length"`
focal length of combination of convex and cocave lenses is given by
`1/F=1/(f_(1))=1/F_(2)`
Where, `f_(1)` and `f_(2)` be the focal lengths of convex and concave lenses, respectively
Now, `1/F=1/0.4=1/((-0.25))`
`1/F=-1.5`
Power, P=-1.5D