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

Answer

B

Explanation

Power of the lens combination is
`P=P_(1)+P_(2)=(1)/(f_(1)("in m"))+(1)/(f_(2)("in m"))=(1)/(+0.40"m")+(1)/(-0.25"m")`
` " "=-1.5 "m"^(-1)= -1.5 D.`